ARM: omap3: overo: Fix MMC init for SPL

The SPL for the Overo board does not initialise the MMC. Hence, it
cannot load the main boot loader from the SD card susequently. This
patch moves the initialisation code for the MMC so it gets included in
the SPL.

[trini: Add missing header]
Signed-off-by: Tom Rini <trini@konsulko.com>