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>