avr32: Provide a way to deselect pins in the portmux

Currently, setting up the portmux is completely one-shot: Once a pin is
muxed, the portmux driver will complain loudly and refuse to do anything
if you try to set up the same pin again.

Sometimes, it may be necessary to change the configuration of a pin
after it has been set up initially. This patch adds a way to undo the
previous configuration, allowing the pin to be reconfigured.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>