arm: mach-k3: Add basic support for J784S4 SoC definition

Add J784S4 initialization files for initial SPL boot.

config SYS_K3_MCU_SCRATCHPAD_BASE default value is same
for J721E, J721S2, J784S4. So combined them into a single
default.

Signed-off-by: Hari Nagalla <hnagalla@ti.com>
[ add firewall configurations and change the R5 MCU scratchpad ]
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Signed-off-by: Dasnavis Sabiya <sabiya.d@ti.com>
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com> # AM69-SK