rp2/CMakeLists: Use armv6m mpy-cross arch for rp2.

Commit 0e28a1f0e5 made it possible to set
-march=armv6m.  It needs to be used when freezing for rp2.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/rp2/CMakeLists.txt

@@ -323,7 +323,7 @@ list(APPEND MICROPY_SOURCE_QSTR
 )
 
 # Define mpy-cross flags
-set(MICROPY_CROSS_FLAGS -march=armv7m)
+set(MICROPY_CROSS_FLAGS -march=armv6m)
 
 # Set the frozen manifest file
 if (MICROPY_USER_FROZEN_MANIFEST)