rp2: Add new port to Raspberry Pi RP2 microcontroller.

This commit adds a new port "rp2" which targets the new Raspberry Pi RP2040
microcontroller.

The build system uses pure cmake (with a small Makefile wrapper for
convenience).  The USB driver is TinyUSB, and there is a machine module
with most of the standard classes implemented.  Some examples are provided
in the examples/rp2/ directory.

Work done in collaboration with Graham Sanderson.

Signed-off-by: Damien George <damien@micropython.org>

examples/rp2/pio_exec.py

@@ -0,0 +1,27 @@
+# Example using PIO to turn on an LED via an explicit exec.
+#
+# Demonstrates:
+#   - using set_init and set_base
+#   - using StateMachine.exec
+
+import time
+from machine import Pin
+import rp2
+
+# Define an empty program that uses a single set pin.
+@rp2.asm_pio(set_init=rp2.PIO.OUT_LOW)
+def prog():
+    pass
+
+
+# Construct the StateMachine, binding Pin(25) to the set pin.
+sm = rp2.StateMachine(0, prog, set_base=Pin(25))
+
+# Turn on the set pin via an exec instruction.
+sm.exec("set(pins, 1)")
+
+# Sleep for 500ms.
+time.sleep(0.5)
+
+# Turn off the set pin via an exec instruction.
+sm.exec("set(pins, 0)")