MXSFB: Fix driver registration

The driver should be registered with mxsfb_driver, not with mxsfb_devtype.
This caused obvious null pointer dereference and crash.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Huang Shijie <b32955@freescale.com>
Acked-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>