dt-bindings: usb: Add usb-phy property to the jz4740-musb node

Add a required 'usb-phy' property, to obtain a phandle to the USB PHY
from devicetree.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>