ab8500_charger: Charger current step-up/down

There is no state machine in the AB to step up/down the charger
current to avoid dips and spikes on VBUS and VBAT when charging
is started. Instead this is implemented in SW.

Signed-off-by: Johan Bjornstedt <johan.bjornstedt@stericsson.com>
Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Karl KOMIEROWSKI <karl.komierowski@stericsson.com>