clk: sunxi: pll2: Add A13 support

The A13, unlike the A10 and A20, doesn't use a pass-through exception for
the 0 value in the pre and post dividers, but increments all the values
written in the register by one.

Add an exception for both these cases to handle them nicely.

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