arm: mach-k3: am642: Add support for boot device detection

AM642 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: Keerthy <j-keerthy@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>