arm: K3: am654: Add support for boot device detection

arm: K3: am654: Add support for boot device detection

AM654 allows for booting from primary or backup boot media.
Both media can be chosen individually based on switch settings.
ROM looks for a valid image in primary boot media, if not found
then looks in backup boot media. In order to pass this boot media
information to boot loader, ROM stores a value at a particular
address. Add support for reading this information and determining
the boot media correctly.

Signed-off-by: Lokesh Vutla <[email protected]>
Signed-off-by: Andreas Dannenberg <[email protected]>
Signed-off-by: Vignesh R <[email protected]>
Reviewed-by: Tom Rini <[email protected]>