mfd: Expose TurnOnStatus in ab8500 sysfs

Expose TurnOnStatus (Power key, RTC alarm, Vbus detect, etc) in
sysfs. This magic value can be read by system users to determine
what caused the platform to turn on last (this) time.

Signed-off-by: Andrew Lynn <andrew.lynn@stericsson.com>
Reviewed-by: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Reviewed-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>