avr32: Add support for "GPIO" port mux

The "GPIO" port mux is used on AVR32 UC3 parts as well as AT32AP7200 and
all later AVR32 chips. This patch adds a driver for it, implementing the
same API as the existing portmux-pio driver but with more functionality.

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