ST USB-PHY controllers

The DWC3 port#2 can be used with a MiPHY2 to manage USB3 devices.

This is to document the DT parameters to be passed from the blob in that case.

Required properties:

MiPHY2:
- compatible : should be "st,sti-usb3phy"
- reg : base physical address of the phy registers and length of memory mapped
	region.
	It must have: "usb3-uport", "pipew";
	So MiPHY and pipe wrapper.
- resets: to reset the device.
- clocks: reference clk to usb3 phy
- clock-names: clock name of usb3 phy reference clock.
- st,fvco-ppm: MIPHY PLL FVCO tolerance in PPM.

Optional property:
- st,auto-calibration: boolean property to advice system to perform
		       autocalibration on device disconnection.
		       This will allow to setup the proper callback in the
		       phy-st-usb3.c source. To setup only for those socs that
		       support autocalibration.
- st,no-ssc: boolean property to disable ssc modulation at miphy setup time.
		       The are spare hard disks that don't support it and
		       so they result unusable due to continuous electrical
		       problems at runtime.

Example:

/* miPHY2 */
usb3_phy: usb3_phy@0 {
	compatible	= "st,sti-usb3phy;
	#phy-cells	= <0>;
	resets		= <&softreset STIH407_MIPHY2_SOFTRESET>;
	reg		= <0x08f90000 0xff>, <0x08f95000 0xff>;
	reg-names	= "usb3-uport", "pipew";
	clocks = <&CLK_SYSIN>;
	clock-names = "miphy_osc";
	st,fvco-ppm = <0>;
	st,auto-calibration;
};
