dt-bindings: iio: adc: Add buffered input property

This patch adds the buffered mode device tree property 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>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>