power: step-chg-jeita: Implement tapered down step charging

Add support to gradually ramp down FCC in steps of pre-configured
value for step-charging implementation in order to approximate
CC1->CV1->CC2->CV2 behaviour.

Change-Id: Ie7dae3a551abc0a17dcfa19c2a258dd12ce4d877
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>