drivers/iio/adc/ad9361: Misc changes (WIP)

Add Slow Attack Gain Control (untested)

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Merge branch 'xcomm_zynq_new_pcore_regmap' of https://github.com/analogdevicesinc/linux into xcomm_zynq_new_pcore_regmap