ASoC: tegra: introduce separate Kconfig variable for DAS driver

This is mainly for symmetry with a future Tegra30 driver, where the
equivalent of the DAS (the AHUB) is useful separately from the I2S driver.

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