clk: meson8b: export the USB clocks

Export the USB related clocks (for the USB controller and the USB2 PHYs)
so they can be used in the dt-bindings.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>