mfd: AB8500 debugfs

This patch adds the possibility to read and write registers
via the debug_fs. It also adds ranges of registers sorted by bank
which makes it possible to read all defined registers in a bank.

Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>