machine/stm32: add OTG FS USB driver for F4/F7

STM32F4 and F7 share the same OTG FS IP but have no USB driver in
TinyGo. This adds a full device-mode driver covering CDC, HID, and
MSC with working examples.

- OTG FS has 4 physical EPs (0–3); virtual indices 4–7 fold onto
  them via physEP() so the existing machine/usb API is unchanged
- F4 bypasses VBUS sensing via GCCFG.NOVBUSSENS; F7 uses the USB
  voltage regulator + GOTGCTL B-valid override instead
- STM32F7 PLL_Q changed 2→9 to produce the 48 MHz clock required
  by USB/RNG/SDMMC; CK48MSEL cleared to select main PLL as source
- HID and MSC descriptors remapped at init() to physical endpoint
  addresses (EP2/EP1 for HID, EP2/EP3 for MSC)
- usb-storage example replaced machine.Flash with a FAT12 RAM disk
  so the host mounts without reformatting
- MSC sendCSW sets queuedBytes before state transition to fix a
  missed byte-count on the status phase

src/examples/hid-keyboard/main.go

@@ -22,6 +22,7 @@ func main() {
 	button.Configure(machine.PinConfig{Mode: machine.PinInputPullup})
 
 	kb := keyboard.Port()
+	machine.USBDev.Configure(machine.UARTConfig{}) // no-op if already init'd by serial.usb
 
 	for {
 		if !button.Get() {

src/examples/usb-storage/main.go

@@ -6,8 +6,85 @@ import (
 	"time"
 )
 
+// Disk geometry.
+const (
+	sectorSize  = 512
+	diskSectors = 128 // 64 KB total
+)
+
+var diskData [diskSectors * sectorSize]byte
+
+type ramDisk struct{}
+
+func (r *ramDisk) ReadAt(p []byte, off int64) (int, error) {
+	return copy(p, diskData[off:]), nil
+}
+
+func (r *ramDisk) WriteAt(p []byte, off int64) (int, error) {
+	return copy(diskData[off:], p), nil
+}
+
+func (r *ramDisk) Size() int64                        { return int64(diskSectors * sectorSize) }
+func (r *ramDisk) WriteBlockSize() int64              { return sectorSize }
+func (r *ramDisk) EraseBlockSize() int64              { return sectorSize }
+func (r *ramDisk) EraseBlocks(start, len int64) error { return nil }
+
+func init() {
+	formatFAT12(diskData[:])
+}
+
+// formatFAT12 writes a minimal FAT12 volume boot record and FAT tables so the
+// host OS can mount the disk without reformatting.
+func formatFAT12(d []byte) {
+	// --- Sector 0: Volume Boot Record ---
+	s := d[0:]
+	s[0] = 0xEB
+	s[1] = 0x3C
+	s[2] = 0x90 // short JMP + NOP
+	copy(s[3:11], "MSDOS5.0")
+	// BPB fields (little-endian)
+	s[11] = 0x00
+	s[12] = 0x02 // bytesPerSector = 512
+	s[13] = 0x01 // sectorsPerCluster = 1
+	s[14] = 0x01
+	s[15] = 0x00 // reservedSectors = 1
+	s[16] = 0x02 // numFATs = 2
+	s[17] = 0x20
+	s[18] = 0x00 // rootEntryCount = 32
+	s[19] = 0x80
+	s[20] = 0x00 // totalSectors16 = 128
+	s[21] = 0xF8 // mediaType = fixed disk
+	s[22] = 0x01
+	s[23] = 0x00 // sectorsPerFAT = 1
+	s[24] = 0x80
+	s[25] = 0x00 // sectorsPerTrack = 128
+	s[26] = 0x01
+	s[27] = 0x00 // numHeads = 1
+	// hiddenSectors[28:32] = 0
+	// totalSectors32[32:36] = 0
+	s[38] = 0x29 // extBootSig
+	s[39] = 0x47
+	s[40] = 0x4F
+	s[41] = 0x30
+	s[42] = 0x31                  // volumeID "GO01"
+	copy(s[43:54], "TINYGO     ") // volumeLabel (11 bytes)
+	copy(s[54:62], "FAT12   ")    // fsType
+	s[510] = 0x55
+	s[511] = 0xAA // boot sector signature
+
+	// --- Sector 1: FAT1 ---
+	// Entry 0 = 0xFF8 (media byte), entry 1 = 0xFFF (EOC); all others = free.
+	d[512] = 0xF8
+	d[513] = 0xFF
+	d[514] = 0xFF
+
+	// --- Sector 2: FAT2 (identical copy) ---
+	copy(d[1024:1027], d[512:515])
+}
+
 func main() {
-	msc.Port(machine.Flash)
+	msc.Port(&ramDisk{})
+	machine.USBDev.Configure(machine.UARTConfig{})
 
 	for {
 		time.Sleep(2 * time.Second)