mfd: AB8500 system control driver

This adds a pretty straight-forward system control driver for the
AB8500. This driver will be used from the core platform, e.g the
clock tree implementation in the machine code, and is by nature
singleton.

There are a few simple functions to read, write, set and clear
registers so that the machine code can control its own foundation.

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