power_supply: add CC_STEP and CC_STEP_SEL properties

There are several algorithms which modify the charge current in steps
across the charge cycle. CC_STEP is used to notify of all the upcoming
charge currents.

The CC_STEP_SEL property is used to select the index of the CC_STEP to
read or modify using the CC_STEP property.

Change-Id: Ieeb533b758035c1c408cdfd68f001374bf0987a5
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>