ASoC: tegra: add clk_prepare/clk_unprepare

Use clk_prepare/clk_unprepare as required by the generic clk framework.

Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>