arm: spear6xx: add DT description of the ADC on SPEAr600

The SPEAr600 has a built-in ADC, which already has a Device Tree binding
described in
Documentation/devicetree/bindings/staging/iio/adc/spear-adc.txt. This
commit adds the description in the SPEAr600 Device Tree of this ADC
device.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>