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>