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

Add binding to specify Spread Spectrum Clocking mode used.

Signed-off-by: Swapnil Jakhade <sjakhade@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211223060137.9252-5-sjakhade@cadence.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>