OMAP3: MMC: Add mux for pins

For OMAP3 add MMC1 MMC2 pin mux MMC3 mux is not added as there are
multiple configurations possible, so the muxing is left to be done
in board file.

Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Chikkature Rajashekar <madhu.cr@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>