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

This patch adds the dt documentation for the usb picophy found on stih407 SoC family
available from STMicroelectronics.
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/phy-stih407-usb.txt

+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";
+};