ASoC: AD1836: Add input gain control for ADC2

The AD1836 has a PGA for its second ADC. This patch adds a control for
adjusting the the gain of the PGA.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>