ti816x: Add additional boot device detection logic

It has been observed that between PG1.0 and PG2.0/2.1 depending on
which device we boot from, we may see a different value here than is
documented in the TRM.  Update the values for NAND and MMC1 based on
real life usage on each revision.

Signed-off-by: Tom Rini <trini@konsulko.com>