spl: mmc: add support for BOOT_DEVICE_MMC2

Currently the mmc device that SPL looks at is always mmc0, regardless
of the BOOT_DEVICE_MMCx value. This forces some boards to
implement hacks in order to boot from other mmc devices.

Make SPL take into account the correct mmc device.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>