dt-binding: usb: add bindings for some common properties

Add bindings for common properties, include maximum-speed,
dr_mode and phy_type

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

doc/device-tree-bindings/usb/generic.txt

@@ -0,0 +1,31 @@
+Generic USB Properties
+
+Optional properties:
+ - maximum-speed: tells USB controllers we want to work up to a certain
+			speed. Valid arguments are "super-speed-plus",
+			"super-speed", "high-speed", "full-speed" and
+			"low-speed". In case this isn't passed via DT, USB
+			controllers should default to their maximum HW
+			capability.
+ - dr_mode: tells Dual-Role USB controllers that we want to work on a
+			particular mode. Valid arguments are "host",
+			"peripheral" and "otg". In case this attribute isn't
+			passed via DT, USB DRD controllers should default to
+			OTG.
+ - phy_type: tells USB controllers that we want to configure the core to support
+			a UTMI+ PHY with an 8- or 16-bit interface if UTMI+ is
+			selected. Valid arguments are "utmi" and "utmi_wide".
+			In case this isn't passed via DT, USB controllers should
+			default to HW capability.
+
+This is an attribute to a USB controller such as:
+
+dwc3@4a030000 {
+	compatible = "synopsys,dwc3";
+	reg = <0x4a030000 0xcfff>;
+	interrupts = <0 92 4>
+	usb-phy = <&usb2_phy>, <&usb3,phy>;
+	maximum-speed = "super-speed";
+	dr_mode = "otg";
+	phy_type = "utmi_wide";
+};