configs: j721e_sk: Move to separate defconfig for J721E SK board

Add defconfig for J721E SK R5 and A72 configuration.

This includes and modifies the J721E EVM defconfigs:
j721e_evm_r5_defconfig -> j721e_sk_r5_defconfig
j721e_evm_a72_defconfig -> j721e_sk_a72_defconfig

Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
diff --git a/board/ti/j721e/MAINTAINERS b/board/ti/j721e/MAINTAINERS
index f5ca7d0..06aba53 100644
--- a/board/ti/j721e/MAINTAINERS
+++ b/board/ti/j721e/MAINTAINERS
@@ -5,5 +5,7 @@
 F:	include/configs/j721e_evm.h
 F:	configs/j721e_evm_r5_defconfig
 F:	configs/j721e_evm_a72_defconfig
+F:	configs/j721e_sk_r5_defconfig
+F:	configs/j721e_sk_a72_defconfig
 F:	configs/j7200_evm_r5_defconfig
 F:	configs/j7200_evm_a72_defconfig