power: qpnp-fg-gen3: Reorganize the FG driver

To help with GEN4 FG driver, re-organize GEN3 FG driver so that
the common code can be re-used. This also involves in updating
the data structures used for GEN3 FG driver; fg_chip is currently
used as the top level data structure and that is changed to a
common data structure fg_dev with fg_gen3_dev replacing it as top
level data structure.

Also, some other definitions which are local only to GEN3 FG are
moved to qpnp-fg-gen3.c and other utility files accordingly. In
addition, fg-memif.c is modified in order to support the GEN4
FG SRAM.

Change-Id: I72acc8c83bca6b82af68cff110857882095b2668
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>