armv7R: K3: j721e: Add support for boot device detection

J721E 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 <lokeshvutla@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>