clk: sunxi: Implement A31 USB clock

The A31 USB clock slightly differ from its older counterparts, mostly
because it has a different gate for each PHY, while the older one had
a single gate for all the phy.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Emilio López <emilio@elopez.com.ar>