simplefb: add support for a8b8g8r8 pixel format

A framebuffer of this format is set up by SHIELD's bootloader.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>