ASoC: davinci: Kconfig: Update the edma-pcm section's dependency and help

Instead of depending on individual SoCs make the edma-pcm depend on the eDMA
dmaengine driver (TI_EDMA).
Update the help text and add DRA7xx family since they have eDMA integrated
as well along with sDMA.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>