video: imxfb: Add DT support

Add devicetree support for imx framebuffer driver. It uses the generic
display bindings and helper functions.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>