mfd: Break out ab5500 debugfs code

This breaks the debugfs portions of the AB5500 driver into its own
file. Split off a _raw function to access registers since we don't
want to expose a generically named function globally. Move all
required data structures to a shared ab5500-core.h file.

Cc: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>