Documentation: dt-bindings: Add documentation for the Meson USB2 PHYs

Add the documentation for the bindings for the Meson8b and GXBB USB2
PHYs.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>