add rp2040, pico

adds preliminary support (just enough to run blinky1) for the Raspberry Pi Pico board along with the rp2040 mcu.

src/machine/machine_rp2040_watchdog.go

@@ -0,0 +1,27 @@
+// +build rp2040
+
+package machine
+
+import (
+	"device/rp"
+	"runtime/volatile"
+	"unsafe"
+)
+
+type watchdogType struct {
+	ctrl    volatile.Register32
+	load    volatile.Register32
+	reason  volatile.Register32
+	scratch [8]volatile.Register32
+	tick    volatile.Register32
+}
+
+var watchdog = (*watchdogType)(unsafe.Pointer(rp.WATCHDOG))
+
+// startTick starts the watchdog tick.
+// cycles needs to be a divider that when applied to the xosc input,
+// produces a 1MHz clock. So if the xosc frequency is 12MHz,
+// this will need to be 12.
+func (wd *watchdogType) startTick(cycles uint32) {
+	wd.tick.Set(cycles | rp.WATCHDOG_TICK_ENABLE)
+}