staging:iio:adc: Add SPEAr ADC driver

This patch implements the basic single data conversion support for
the SPEAr600 SoC ADC. The register layout of SPEAr600 differs a bit
from other SPEAr SoC variants (e.g. SPEAr3xx). These differences are
handled via DT compatible testing. Resulting in a multi-arch binary.

This driver is currently tested only on SPEAr600. Future patches may add
support for other SoC variants (SPEAr3xx) and features like software
buffer or DMA.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@st.com>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>