ASoC: tegra: Use flat regcache

When using an rbtree cache, there can be allocations the first time a
register is accessed.  This can cause an attempt to schedule while
atomic in the case that the regmap is using a spinlock.  This could be
fixed by either initializing all the registers or using a flat cache.
The register maps for tegra30_ahub and tegra30_i2s are dense and don't
save much from using a tree so convert them to flat.

Tegra30 changes tested on Norrin, Tegra20 changes compile.

Signed-off-by: Dylan Reid <dgreid@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>