ARMV7: OMAP4: Enable saveenv to eMMC for SDP4430

The SDP4430 does not have onboard NAND, it has eMMC on the second
MMC slot.  This patch adds support for saving the u-boot environment
to eMMC.

Signed-off-by: Aneesh V <aneesh@ti.com>
Signed-off-by: Sukumar Ghorai <s-ghorai@ti.com>
Tested-by: Steve Sakoman <steve@sakoman.com>