mfd: ab8500-sysctrl.c: Provide charging as reset reason

It's possible to supply a string to provide a reason for
triggering a restart. In this case our reason is to enter
charging-only mode, as a charger was found to be present.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Reviewed-by: Karl KOMIEROWSKI <karl.komierowski@stericsson.com>