clk: sunxi-ng: Implement multiplier maximum

Some multipliers have a maximum rate that is lower than what the register
width allows to. Add a field in the multiplier structure to allow CCU
driver to set that maximum.

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