drm: atmel-hlcdc: add support for 8-bit color lookup table mode

All layers of all supported chips support this, the only variable is the
base address of the lookup table in the register map.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1498107791-17450-3-git-send-email-peda@axentia.se