ASoC: tegra: move AC97 clock handling to the machine driver

On Tegra the convention is to have a single machine driver
that's controlling the whole audio subsystem.

Move the clock handling to the machine driver, to be in line
with the other Tegra drivers and give the machine driver
full control over the single Tegra audio PLL.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>