power: vm-bms: Add charge_counter property

Add charge_counter property to vm-bms which is expressed
in terms of battery FCC and SOC so that the clients can
make use of it as needed.

CRs-Fixed: 2188284
Change-Id: I7bc732f8b9b4125f8d2ad4d5e3c8d9b50a27350e
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>