avr32: add pin mask for 18-bit color on the LCD controller

This patch adds two defines for setting a pin mask for 18-bit LCD panels
connected to the LCD controller. One mask for primary output and one
mask for alternative output.

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