mfd: Reentrance and revamp ab8500 gpadc fetching interface

This revamps the interface so that AB8500 GPADCs are fetched by
name. Probed GPADCs are added to a list and this list is searched
for a matching GPADC. This makes it possible to have multiple
AB8500 GPADC instances instead of it being a singleton, and
rids the need to keep a GPADC pointer around in the core AB8500
MFD struct.

Currently the match is made to the device name which is by default
numbered from the device instance such as "ab8500-gpadc.0" but
by using the .init_name field of the device a more intiutive
naming for the GPADC blocks can be achieved if desired.

Signed-off-by: Daniel Willerud <daniel.willerud@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>