offb: Add palette hack for little endian

The pseudo palette color entries need to be ajusted for little
endian.

This patch byteswaps the values in the pseudo palette depending
on the host endian order and the screen depth.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>