power: qpnp-fg: support reading ESR count

Support reading the fuel gauge ESR count in order to be able to tell
whether a software forced ESR pulse is needed.

CRs-Fixed: 750051
Change-Id: Ida096775798f3479da1f6a2a9664407724e93325
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>