avr32: MRMT: correct setup of SPI slaves

For MRMT1/2 add-on card to Atmel ATNGW100.

This patch implements the SPI slave setup method created by patch:
<<avr32: function for independently setting up SPI slaves>>

Signed-off-by: Peter Ma <pma@mediamatech.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>