configs: am69_sk: Add defconfig for AM69 SK board

Add defconfig for AM69 SK A72 and R5 configuration.

This inlcudes and modifies the J784S4 EVM defconfigs:
j784s4_evm_a72_defconfig -> am69_sk_a72_defconfig
j784s4_evm_r5_defconfig -> am69_sk_r5_defconfig

Signed-off-by: Dasnavis Sabiya <sabiya.d@ti.com>
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com> # AM69-SK