omap: init the gpio pinmux for mmc

There is two gpio for mmc use, one is for card detecting, another is
used for checking write protect. Intialize its pinmux in case the bootloader
doesn't set it.

Signed-off-by: Stanley.Miao <stanley.miao@windriver.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>