atmel_lcdfb: support new-style palette format

The newer Atmel SoCs use normal 16bit 565 BGR/RGB for the palette data,
rather than the special intensity + 555 format.

Fill out palette data correctly on these devices, and at the same time
respect the RGB/BGR wiring mode.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>