phy: phy-stih407-usb: Add dt documentation for USB picophy found on stih407 SoC family

ST STiH407 USB PHY controller

This file documents the dt bindings for the usb picoPHY driver which is the PHY for both USB2 and USB3

host controllers (when controlling usb2/1.1 devices) available on STiH407 SoC family from STMicroelectronics.

Required properties:

- compatible		: should be "st,stih407-usb2-phy"

- reg			: contain the offset and length of the system configuration registers

			  used as glue logic to control & parameter phy

- reg-names		: the names of the system configuration registers in "reg", should be "param" and "reg"

- st,syscfg		: sysconfig register to manage phy parameter at driver level

- resets		: list of phandle and reset specifier pairs. There should be two entries, one

			  for the whole phy and one for the port

- reset-names		: list of reset signal names. Should be "global" and "port"

See: Documentation/devicetree/bindings/reset/st,sti-powerdown.txt

See: Documentation/devicetree/bindings/reset/reset.txt

Example:

usb2_picophy0: usbpicophy@f8 {

	compatible	= "st,stih407-usb2-phy";

	reg		= <0xf8 0x04>,	/* syscfg 5062 */

			  <0xf4 0x04>;	/* syscfg 5061 */

	reg-names	= "param", "ctrl";

	#phy-cells	= <0>;

	st,syscfg	= <&syscfg_core>;

	resets		= <&softreset STIH407_PICOPHY_SOFTRESET>,

			  <&picophyreset STIH407_PICOPHY0_RESET>;

	reset-names	= "global", "port";

};