configs: j784s4_evm: Add defconfig for J784S4 EVM board

Add defconfigs for building R5 U-Boot SPL and A72 U-Boot.

Signed-off-by: Hari Nagalla <hnagalla@ti.com>
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com> # AM69-SK