Add a driver to control the clock divider found in the sample clock
generator of the axg audio clock controller.

The sclk divider accumulates specific features which make the generic
divider unsuitable to control it:
- zero based divider (div = val + 1), but zero value gates the clock,
  so minimum divider value is 2.
- lrclk variant may adjust the duty cycle depending the divider value
  and the 'hi' value.
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>