clk: mediatek: mt7629: add support for ssusbsys

The SSUSB IP's clocks come from ssusbsys module on mt7629,
so add its driver

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>