video: mxsfb: add simple device tree probe

Add a simple device tree probe support for mxsfb driver.  Before
a common binding for struct fb_videomode is available, the driver will
keep using struct mxsfb_platform_data.  That said, platform code will
use auxdata to attach mxsfb_platform_data for device tree boot.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>