power_supply: add two properties for step charging

STEP_CHARGING_ENABLED indicates if step charging is enabled or not.
STEP_CHARGING_STEP reflects step charging phase.

Both properties are added for the purpose of testing and debugging.

CRs-Fixed: 1052854
Change-Id: I7d7517ab3f172016bb2f1c23fb2788bbd2d01086
Signed-off-by: Harry Yang <harryy@codeaurora.org>