ASoC: sgtl5000: Let the codec acquire its clock

On a mx6qsabrelite board the following error happens on probe:

sgtl5000: probe of 0-000a failed with error -5
imx-sgtl5000 sound.13: ASoC: CODEC (null) not registered
imx-sgtl5000 sound.13: snd_soc_register_card failed (-517)
platform sound.13: Driver imx-sgtl5000 requests probe defer

Prior to reading the codec ID we need to turn the SYS_MCLK clock, so let's
enable the codec clock inside sgtl5000_i2c_probe().

Also remove the codec clock enable/disable functions from the machine driver.

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