video: cfb_console: add weak default video_set_lut()

Do not enforce drivers to provide empty video_set_lut()
if they do not implement indexed color (8 bpp) frame
buffer support. Add default function to the cfb_console
driver and remove empty video_set_lut() functions.

Signed-off-by: Anatolij Gustschin <agust@denx.de>