mfd: ab8500: Augment DT bindings

As we migrate the AB8500 GPADC driver to use IIO, we need to augment
the bindings to account for defining the ADC channels in the device
tree.

Cc: devicetree@vger.kernel.org
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>