ARM: sun6i: Add the USB clocks to the DTSI

The USB clocks of the A31 seems to be parented to the 24MHz oscillator, and
handle the clocks for the USB phys and OHCI devices.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>