clk: sunxi: Add sun8i MBUS clock support

The MBUS clock on sun8i is slightly different from the old mod0 clocks.
The divider is 3 bits wider, while also needing a divider table for the
higher 4 values, which all set the same divider.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>