power: smb2: expose CHARGE_FULL/CYCLE_COUNT properties

Expose CHARGE_FULL/CYCLE_COUNT properties via "battery"
power_supply class.

While at it, create a common function to read properties
from "bms" power_supply_class.

Change-Id: I377ecea6e62c1eb8716982b79ec4c236c14bc2d5
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>