mfd: ab8500-debugfs: Formated access AB8500 registers from debugfs entry

Add debugfs entry ab8500/hwreg to read/write bit-field in AB8500 registers.
Check the debugfs entries usage from heading comments in ab8500-debugfs.c

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: carriere etienne <etienne.carriere@stericsson.com>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>