dt-bindings: usb: amlogic,meson-g12a-usb-ctrl: add the Amlogic GXL and GXM Families USB Glue Bindings

The Amlogic GXL and GXM is slightly different from the Amlogic G12A Glue.

The GXL SoCs only embeds 2 USB2 PHYs and no USB3 PHYs, and the GXM SoCs
embeds 3 USB2 PHYs.
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>