avr32: Allow selecting multiple pins at once

at32_select_periph() now takes an u32 bitmask rather than a single pin.
This allows to set multiple pins at once.

Signed-off-by: Alex Raimondi <mailinglist@miromico.ch>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>