mfd: Switch the AB8500 GPADC to IIO

The AB8500 GPADC driver is indeed a "general purpose ADC" driver,
and while the IIO subsystem did not exist when the driver was
first merged, it is never too late to clean things up and move it
to the right place.

Nowadays IIO provides the right abstractions and interfaces to
do generic ADC work in the kernel.

We have to cut a bunch of debugfs luggage to make this transition
swift, but all these files to is read out the raw values of the
ADC and the IIO subsystem already has a standard sysfs ABI for
doing exactly this: no debugfs is needed.

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>