configs: Add defconfig for USB DFU bootmode

Because of space constraints, create a new USB defconfig for R5 to
faciliate booting in USB peripheral (DFU) bootmode

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
diff --git a/board/ti/am65x/MAINTAINERS b/board/ti/am65x/MAINTAINERS
index 7c52e75..c9e0871 100644
--- a/board/ti/am65x/MAINTAINERS
+++ b/board/ti/am65x/MAINTAINERS
@@ -5,3 +5,4 @@
 F:	include/configs/am65x_evm.h
 F:	configs/am65x_evm_a53_defconfig
 F:	configs/am65x_evm_r5_defconfig
+F:	configs/am65x_evm_r5_usbdfu_defconfig