dt-bindings: phy: add support for STM32 USB PHY Controller (USBPHYC)

This patch adds the device tree bindings description for STM32 USBPHYC
(USB PHY Controller).

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>