avr32: Allow reserving multiple pins at once

at32_reserve_pin now takes an u32 bitmask rather than a single pin.
This allows to reserve multiple pins at once.

Remove (undocumented) SDCS (pin PE26) from reservation in board
setup code.

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