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.go

@@ -0,0 +1,44 @@
+// +build rp2040
+
+package machine
+
+import (
+	"device/rp"
+	_ "unsafe"
+)
+
+//go:linkname machineInit runtime.machineInit
+func machineInit() {
+	// Reset all peripherals to put system into a known state,
+	// except for QSPI pads and the XIP IO bank, as this is fatal if running from flash
+	// and the PLLs, as this is fatal if clock muxing has not been reset on this boot
+	// and USB, syscfg, as this disturbs USB-to-SWD on core 1
+	bits := ^uint32(rp.RESETS_RESET_IO_QSPI |
+		rp.RESETS_RESET_PADS_QSPI |
+		rp.RESETS_RESET_PLL_USB |
+		rp.RESETS_RESET_USBCTRL |
+		rp.RESETS_RESET_SYSCFG |
+		rp.RESETS_RESET_PLL_SYS)
+	resetBlock(bits)
+
+	// Remove reset from peripherals which are clocked only by clkSys and
+	// clkRef. Other peripherals stay in reset until we've configured clocks.
+	bits = ^uint32(rp.RESETS_RESET_ADC |
+		rp.RESETS_RESET_RTC |
+		rp.RESETS_RESET_SPI0 |
+		rp.RESETS_RESET_SPI1 |
+		rp.RESETS_RESET_UART0 |
+		rp.RESETS_RESET_UART1 |
+		rp.RESETS_RESET_USBCTRL)
+	unresetBlockWait(bits)
+
+	clocks.init()
+
+	// Peripheral clocks should now all be running
+	unresetBlockWait(RESETS_RESET_Msk)
+}
+
+//go:linkname ticks runtime.machineTicks
+func ticks() uint64 {
+	return timer.timeElapsed()
+}