avr32: fix 15-bit LCDC pin mask to use MSB lines

This patch corrects the 15-bit LCDC pin mask definitions to select the
five upper lines in each color byte from the LCDC data output. When
reducing the color depth the LCDC will start filling MSB and downwards.

Also only enable 5 bits per color as the define indicates.

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