dt-bindings: phy: ti: add documentation for ti,dra7x-usb2

Commit 7e472402 ("phy: omap-usb2: Provide workaround for
USB2PHY false disconnect") added a new binding for USB2 PHYs
on DRA7x. But it has remained undocumented so far.

Add documentation for the binding.
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/ti-phy.txt

@@ -31,6 +31,8 @@ OMAP USB2 PHY
 
 Required properties:
  - compatible: Should be "ti,omap-usb2"
+	       Should be "ti,dra7x-usb2" for the 1st instance of USB2 PHY on
+	       DRA7x
 	       Should be "ti,dra7x-usb2-phy2" for the 2nd instance of USB2 PHY
 	       in DRA7x
  - reg : Address and length of the register set for the device.