add rp2040, pico

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

src/machine/board_pico.go

@@ -0,0 +1,43 @@
+// +build pico
+
+package machine
+
+// GPIO pins
+const (
+	GP0  Pin = 0
+	GP1  Pin = 1
+	GP2  Pin = 2
+	GP3  Pin = 3
+	GP4  Pin = 4
+	GP5  Pin = 5
+	GP6  Pin = 6
+	GP7  Pin = 7
+	GP8  Pin = 8
+	GP9  Pin = 9
+	GP10 Pin = 10
+	GP11 Pin = 11
+	GP12 Pin = 12
+	GP13 Pin = 13
+	GP14 Pin = 14
+	GP15 Pin = 15
+	GP16 Pin = 16
+	GP17 Pin = 17
+	GP18 Pin = 18
+	GP19 Pin = 19
+	GP20 Pin = 20
+	GP21 Pin = 21
+	GP22 Pin = 22
+	GP23 Pin = 23
+	GP24 Pin = 24
+	GP25 Pin = 25
+	GP26 Pin = 26
+	GP27 Pin = 27
+	GP28 Pin = 28
+	GP29 Pin = 29
+
+	// Onboard LED
+	LED Pin = GP25
+
+	// Onboard crystal oscillator frequency, in MHz.
+	xoscFreq = 12 // MHz
+)