power: pm8921-bms: support fake battery

Provide ability to tell the bms that a device is running on a fake
battery. When told so, the bms will calculate the state of charge
as usual but will always report 53% charge left in the battery.

This helps avoiding shutdowns when the percent charge gets very low
- the charge drawn from the fake battery is equal or greater than
the capacity of the battery.

Change-Id: Iabf32adbb0106bde5985dfd6de6517303b5d1922
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>