clk: meson: axg-audio: use the clk input helper function

Rework the axg audio clock controller to use the new clk-input helper
function.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: fixed pclk input clock name to axg_audio_pclk]
Link: http://lkml.kernel.org/r/20181204165819.21541-3-jbrunet@baylibre.com