arm: mach-k3: j721e_init: Add support for backup boot modes

When the boot of J721E devices using the primary bootmode (configured
via device pins) fails a boot using the configured backup bootmode is
attempted. To take advantage of the backup boot mode feature go ahead
and add support to the J721E init code to determine whether the ROM code
performed the boot using the primary or backup boot mode, and if booted
from the backup boot mode, decode the bootmode settings into the
appropriate U-Boot mode accordingly so that the boot can proceed.

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