dt-bindings: document PX30 usb2phy General Register Files

One of the separate General Register Files contains the registers for
controlling the usb2phy, so add the necessary binding compatible for it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20190917082659.25549-11-heiko@sntech.de