dt-bindings: phy: cadence-torrent: Add binding to specify SSC mode

commit cee47038ae90b898adcb87c58d25100748ba6fec from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

Add binding to specify Spread Spectrum Clocking mode used.

Signed-off-by: Swapnil Jakhade <sjakhade@cadence.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>