arm: K3: am654: Choose MMC boot device based on boot port

arm: K3: am654: Choose MMC boot device based on boot port

For most devices the boot mode maps directly to the boot
device. For MMC this is not the case as we have two MMC
boot modes and two MMC boot devices (ports). Check the
boot port to determine which MMC device was our boot
device. Make this change for both primary and secondary
boot modes.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>