pinctrl: raspberrypi: pico: Add GPIO pinctrl definitions

In most circumstances, GPIO pins identified in devicetree are
assigned to the SIO function at initialization.  However, there
are cases where pin assignments change in alternate pinctrl
configurations.  This commit adds the necessary definitions to
specify GPIO opration in pinctrl.

Signed-off-by: Steve Boylan <stephen.boylan@beechwoods.com>