phy: phy-stih41x-usb: Add dt documentation for USB phy on STiH415/6

STMicroelectronics STiH41x USB PHY binding

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This file contains documentation for the usb phy found in STiH415/6 SoCs from

STMicroelectronics.

Required properties:

- compatible	: should be "st,stih416-usb-phy" or "st,stih415-usb-phy"

- st,syscfg	: should be a phandle of the syscfg node

- clock-names	: must contain "osc_phy"

- clocks	: must contain an entry for each name in clock-names.

See: Documentation/devicetree/bindings/clock/clock-bindings.txt

- #phy-cells	: must be 0 for this phy

See: Documentation/devicetree/bindings/phy/phy-bindings.txt

Example:

usb2_phy: usb2phy@0 {

	compatible	= "st,stih416-usb-phy";

	#phy-cell	= <0>;

	st,syscfg	= <&syscfg_rear>;

	clocks		= <&clk_sysin>;

	clock-names	= "osc_phy";

};