ARM: dts: ux500: declare GPADC IIO ADC channels

This adds the IIO channels for the GPADC after converting it
to using the standard IIO ADC bindings and moving the driver
over to the IIO subsystem. We also add IIO hwmon standard
driver node to support reading channels in a standard manner.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>