usb: phy: Add STi USB2 PHY

This is the generic phy driver for the picoPHY ports
used by USB2/1.1 controllers. It is found on STiH407 SoC
family from STMicroelectronics.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/phy/phy-stih407-usb.txt

@@ -0,0 +1,24 @@
+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"
+- st,syscfg		: phandle of sysconfig bank plus integer array containing phyparam and phyctrl register offsets
+- 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 {
+	compatible	= "st,stih407-usb2-phy";
+	#phy-cells	= <0>;
+	st,syscfg	= <&syscfg_core 0x100 0xf4>;
+	resets		= <&softreset STIH407_PICOPHY_SOFTRESET>,
+			  <&picophyreset STIH407_PICOPHY0_RESET>;
+	reset-names	= "global", "port";
+};