ASoC: tas571x: add input channel mixer for TAS5717/19

Add channel 1 and 2 input mixer registers and the related ALSA mixer
controls for TAS5717/19 chips.

The mixer control coefficients on the chip are linear in the range
-3.99999 to +3.99999, encoded in 3.23 number format.
In this patch the mixer controls are limited to 128 values
from 0.0 to 1.0 in 1/64 steps.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Signed-off-by: Mark Brown <broonie@kernel.org>