dt-bindings: phy: meson-gxl-usb2-phy: Add documentation for the Meson GXL USB2 PHY

This adds the DT binding documentation for the USB2 PHY(s) found in the
Meson GXL and GXM SoCs.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>