armv8: K3: j721e: Add custom MMU support

The A72 U-Boot code loads and boots a number of remote processors
including the C71x DSP, both the C66_0 and C66_1 DSPs, and the various
Main R5FSS Cores. Change the memory attributes for the DDR regions used
by the remote processors so that the cores can see and execute the
proper code.

A separate table based on the current AM65x table is added for J721E SoCs,
since the number of remote processors and their DDR usage will be different
between the two SoC families.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>