dts: bindings: Add yaml files for PHY subsystem

Add yaml file for initial support of USB PHYs.
It adds also yaml files for STM32 USBPHYC and
USB controllers with embedded PHY.

PHY refers to the physical layer which is used
to connect a device to the physical medium e.g.
USB controller.

Origin: original

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>