armv8: mach-k3: am642: Add custom MMU support

Change the memory attributes for the DDR regions used by the remote
processors on AM65x so that the cores can see and execute the proper code.

A separate table based on the previous K3 SoCs is introduced since the
number of remote processors and their DDR usage is different between the
SoC families.

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