power: fg-alg: Add support for TTF and TTE calculation

Add support for Time to full (TTF) and Time to empty (TTE)
calculations in fg-alg.c that can be used by FG and QG
drivers.

CRs-Fixed: 2254530
Change-Id: I28e21da30911a8d879f176ae3ff8736aed807020
Signed-off-by: Vamshi Krishna B V <vbv@codeaurora.org>