clk: sunxi-ng: nm: Add support for sigma-delta modulation

Some of the N-M-style clocks, namely the PLLs, support sigma-delta
modulation to do fractional-N frequency synthesis. This is used in
the audio PLL to generate the exact frequency the audio blocks need.
These frequencies can not be generated with integer N-M factors.

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