clk: sunxi: Introduce mbus compatible

Even though the mbus clock is a regular module clock, given its nature, it
needs to be enabled all the time.

Introduce a new compatible, to differentiate it from the other module clocks.

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