rockchip: mkimage: Add support rk3188 serial

Add the entry for the rk3188 requiring rc4-encryption of the SPL.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>