ARM: mx6: add MMC2 boot device detection support in SPL

Check BOOT_CFG2[3:4] to determine which SD/MMC port is selected to boot
from. If MMC2 is selected return BOOT_DEVICE_MMC2. In all other cases
return BOOT_DEVICE_MMC1, as we do not have corresponding macro for MMC3
and MMC4.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>