configs: Add new config for supporting USB mass storage boot

Because of space constraints, create a new USB defconfig for R5 to
faciliate booting from USB mass storage devices

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
diff --git a/board/ti/am65x/MAINTAINERS b/board/ti/am65x/MAINTAINERS
index c9e0871..6da4182 100644
--- a/board/ti/am65x/MAINTAINERS
+++ b/board/ti/am65x/MAINTAINERS
@@ -6,3 +6,4 @@
 F:	configs/am65x_evm_a53_defconfig
 F:	configs/am65x_evm_r5_defconfig
 F:	configs/am65x_evm_r5_usbdfu_defconfig
+F:	configs/am65x_evm_r5_usbmsc_defconfig