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() {