rp2040: use DMA for send-only SPI transfers

This improves slightly. It also is some groundwork for better DMA
support in TinyGo in the future.

I'm not entirely sure why it improves performance (in theory the old
code should already saturate the SPI bus) but it does, so 🤷

src/machine/machine_rp2040.go

@@ -4,6 +4,7 @@ package machine
 
 import (
 	"device/rp"
+	"runtime/volatile"
 	"unsafe"
 )
 
@@ -114,3 +115,23 @@ func ChipVersion() uint8 {
 	version := (chipID & SYSINFO_CHIP_ID_REVISION_BITS) >> SYSINFO_CHIP_ID_REVISION_LSB
 	return uint8(version)
 }
+
+// Single DMA channel. See rp.DMA_Type.
+type dmaChannel struct {
+	READ_ADDR   volatile.Register32
+	WRITE_ADDR  volatile.Register32
+	TRANS_COUNT volatile.Register32
+	CTRL_TRIG   volatile.Register32
+	_           [12]volatile.Register32 // aliases
+}
+
+// Static assignment of DMA channels to peripherals.
+// Allocating them statically is good enough for now. If lots of peripherals use
+// DMA, these might need to be assigned at runtime.
+const (
+	spi0DMAChannel = iota
+	spi1DMAChannel
+)
+
+// DMA channels usable on the RP2040.
+var dmaChannels = (*[12]dmaChannel)(unsafe.Pointer(rp.DMA))