power_supply: add local extension property ESR_COUNT

Add the power supply property POWER_SUPPLY_PROP_ESR_COUNT to denote the
number of successful resistance estimates that the fuel gauge power
supply has read.

CRs-Fixed: 750051
Change-Id: I87d74a083d6149b9d1ca9a8ae1c464225705c607
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>