iio: adc: New driver for the AB8500 GPADC

This is a new driver for the ST-Ericsson AB8500 GPADC, which
replaces the old driver in drivers/mfd/ab8500-gpadc.c and
thus gets rid of another necessarily different custom driver
from the times before IIO existed.

The AB8500 GPADC can convert 10 different channels and these
are used for monitoring voltages in the U8500 chipset, some
are used for battery charging, some for temperature
monitoring.

As this is very core functionality that a lot of drivers
depend on and was formerly compiled in with the AB8500 core
driver, we deafault it to 'y' in Kconfig: it can be compiled
out but it is really not advisible: the platform can
for example overheat if we do.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>