iio: adc: add driver for Rockchip saradc

The ADC is a 3-channel signal-ended 10-bit Successive Approximation
Register (SAR) A/D Converter. It uses the supply and ground as its reference
and converts the analog input signal into 10-bit binary digital codes.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>