i.MX Framebuffer: add TFT support

With TFTs we can do 5/6/5 instead of 4/4/4. Add a bitfield for this
and use it with TFTs.

Acked-by: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>