drm: Add new driver for MXSFB controller

Add new driver for the MXSFB controller found in i.MX23/28/6SX .
The MXSFB controller is a simple framebuffer controller with one
parallel LCD output. Unlike the MXSFB fbdev driver that is used
on these systems now, this driver uses the DRM/KMS framework.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>