Add support for digital volume control.  There is no dedicated register
for volume control but instead there are 4.  The values of the registers
are determined with exponential floating point math.
So a table was created with register values for 2dB step increments
from -110dB to 0dB.
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200221124151.8774-1-dmurphy@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>