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
diff --git a/board/ti/j784s4/MAINTAINERS b/board/ti/j784s4/MAINTAINERS
index 2b6bff2..46f4ba5 100644
--- a/board/ti/j784s4/MAINTAINERS
+++ b/board/ti/j784s4/MAINTAINERS
@@ -16,3 +16,5 @@
 S:	Maintained
 F:	arch/arm/dts/k3-am69-sk-u-boot.dtsi
 F:	arch/arm/dts/k3-am69-r5-sk.dts
+F:	configs/am69_sk_r5_defconfig
+F:	configs/am69_sk_a72_defconfig