avr32: set pin mask to alternative 18 bpp for EVKLCD10x boards

This patch will set the pin mask to alternative 18 bits per pixel output
for EVKLCD10x boards.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>