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

The goal of this driver is to coherently control the phase provided to
the different element using the sample clock generator. This simplify
the usage of the sample clock generator a lot, without comprising the
ability of the SoC.
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>