power: pm8921-bms: expose start and end charging parameters

The userspace needs to know percent, open circuit voltage and columb
based charge properties at the start and end of charging. These values
are essential in calculating the charge delta and possibly generating
an error indication to the user if the battery doesn't hold enough
charge.

CRs-Fixed: 320216
Change-Id: Ida66a62ae9806a7d12b70914d4875ee75d140e5f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>