ws2812: support thingplus-rp2040 board

Added 125 MHz rp2040 timing
  Added unsafe.Pointer for pointer conversion

examples/ws2812/thingplus-rp2040.go

@@ -0,0 +1,13 @@
+//go:build thingplus_rp2040
+// +build thingplus_rp2040
+
+package main
+
+import "machine"
+
+// This is the pin assignment for the internal neopixel of the
+// Sparkfun thingplus rp2040.
+// Replace neo and led in the code below to match the pin
+// that you are using if different.
+var neo machine.Pin = machine.GPIO8
+var led = machine.LED