More silkscreen labels for pico2-ice board

Reading the schematic and the rev2 board viewer:

https://raw.githubusercontent.com/tinyvision-ai-inc/pico2-ice/refs/heads/main/Board/Rev2/bom/ibom.html

concluded that the RP2350B GPIO pins are not labeled with these
pin numbers in the silkscreen. Instead, the silkscreen refers to
uses of the GPIOs or the ICE numbered pins. RP2340B devoted pins
map to the A1..4 B1..4 pins on the RP PMOD connector. Also
silkscreen "~0".."~6" are labeled as pins N0..N6.

Signed-off-by: Tinkerer <tinkerer@zappem.net>

Author: tinkerator

src/machine/board_pico2_ice.go

@@ -59,7 +59,8 @@ const (
 	GP46 Pin = 46
 	GP47 Pin = 47
 
-	// RPi pins shared with ICE
+	// RPi pins shared with ICE. The ICE number is what appears on
+	// the board silkscreen.
 	ICE9  = GP28
 	ICE11 = GP29
 	ICE14 = GP7
@@ -79,13 +80,33 @@ const (
 	ICE35_G0 = GP21
 
 	// Silkscreen & Pinout names
-	ICE_SSN   = ICE16
-	ICE_SO    = ICE14
-	ICE_SI    = ICE17
-	ICE_CK    = ICE15
+	ICE_SSN = ICE16
+	ICE_SO  = ICE14
+	ICE_SI  = ICE17
+	ICE_CK  = ICE15
+	SD      = GP2
+	SC      = GP3
+
 	FPGA_RSTN = GP31
-	ICE_DONE  = GP40
-	USB_BOOT  = GP42
+	A3        = GP32
+	A1        = GP33
+	A4        = GP34
+	A2        = GP35
+	B3        = GP36
+	B1        = GP37
+	B4        = GP38
+	B2        = GP39
+	N0        = GP40 // On the board these are labeled "~0"
+	N1        = GP41
+	N2        = GP42
+	N3        = GP43
+	N4        = GP44
+	N5        = GP45
+	N6        = GP46
+
+	// Functions from Schematic.
+	ICE_DONE = GP40
+	USB_BOOT = GP42
 
 	// Button
 	SW1     = GP42