ST STiH407 USB-PHY controller

This is the driver to provide the picoPHY ports for both USB2 and USB3
Host controllers available on STiH407 SoC families.

On STiH410, we have to use the DWC3 port#2 with a picoPHY to manage USB2
devices. The USB2 Controller will use the ports #0 and #1.

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

Required properties:
- compatible : can 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 filled in "reg".
- st,syscfg: sysconfig register to manage phy parameter at driver level.
- resets, reset-names: to reset the whole phy / port.
- st,compdistune <#>: Disconnect Threshold Adjustment
- st,sqrxtune <#>:Squelch Threshold Adjustment
- st-txfslstune <#>: FS/LS Source Impedance Adjustment
- st,txpreempamptune <#>: HS Transmitter Pre-Emphasis Current Control
- st,txpreemppulsetune <#>: HS Tx Pre-Emphasis Duration Control
- st,txrisetune <#>: HS Tx Rise/Fall Time Adjustment
- st,txvreftune <#>: HS DC Voltage Level Adjustment
- st,txhsxvtune <#>: HS Tx Crossover Adjustment
- st,txrestune <#>: USB Source Impedance Adjustment

Example:
usb2_picophy0: usbpicophy@0 {
	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";
};

usb2_picophy1: usbpicophy@1 {
	compatible = "st,stih407-usb2-phy";
	#phy-cells = <0>;
	reg =	<0xfc 0x04>,	/* syscfg 5063 */
		<0xf4 0x04>;    /* syscfg 5061 */
	reg-names =	"param",
			"ctrl";

	st,syscfg = <&syscfg_core>;
	resets = <&softreset STIH407_PICOPHY_SOFTRESET>,
		 <&picophyreset STIH407_PICOPHY1_RESET>;
	reset-names = "global",
		      "port";
};

usb2_picophy2: usbpicophy@2 {
	compatible = "st,stih407-usb2-phy";
	#phy-cells = <0>;
	reg =	<0x100 0x04>,	/* syscfg 5064 */
		<0xf4 0x04>;    /* syscfg 5061 */
	reg-names =	"param",
			"ctrl";

	st,syscfg = <&syscfg_core>;
	resets = <&softreset STIH407_PICOPHY_SOFTRESET>,
		 <&picophyreset STIH407_PICOPHY2_RESET>;
	reset-names = "global",
		      "port";
	st,txvreftune = <0xb>; /* +16% in HS DC level voltage*/
};
