ASoC: sgtl5000: Add 'clocks' entry as a required propery

Since commit 9e13f345 (ASoC: sgtl5000: Let the codec acquire its clock) , the
'clocks' entry is mandatory, so update the binding doc.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>