configs: rockchip: enable "ums" command for Radxa ROCK 5B

USB Type-C port is configured as "peripheral" port. so enable "ums"
command to use as USB Mass Storage device.
("rockusb" command is already enabled and working)

Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/configs/rock5b-rk3588_defconfig b/configs/rock5b-rk3588_defconfig
index fc118ce..80a2f2f 100644
--- a/configs/rock5b-rk3588_defconfig
+++ b/configs/rock5b-rk3588_defconfig
@@ -39,6 +39,7 @@
 CONFIG_CMD_PCI=y
 CONFIG_CMD_USB=y
 CONFIG_CMD_ROCKUSB=y
+CONFIG_CMD_USB_MASS_STORAGE=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_REGULATOR=y
 # CONFIG_SPL_DOS_PARTITION is not set