iio: adc: ad7124: Add buffered input support

This patch adds the option to enable the buffered mode for positive and
negative inputs. Each option can be enabled independently.

In buffered mode, the input channel feeds into a high impedance input stage
of the buffer amplifier. Therefore, the input can tolerate significant
source impedances and is tailored for direct connection to external
resistive type sensors such as strain gages or RTDs.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>