dt-bindings: phy: phy-stm32-usbphyc: add optional phy tuning properties

This patch adds the description of new optional phy tuning properties
for usbphyc phy sub nodes.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211015161427.220784-3-amelie.delaunay@foss.st.com


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