power: qpnp-fg: add support to configure Rconn

On some platforms, battery connector resistance (Rconn) needs to
be taken into account when Rslow (capacitive reactance) values
are used from battery profile. Without this, battery resistance
observed by FG algorithm will be wrong ending up in wrong SOC
estimate. When Rconn is specified, Rslow values for charge and
discharge will be updated. Add support to configure Rconn through
a device tree parameter "qcom,fg-rconn-mohms".

While at it, fix a bug in reading rs_to_rslow_chg from a wrong
offset 0 instead of 2.

CRs-Fixed: 980218
Change-Id: Ibd93e5013197a91098e3936b1b13f1193789215c
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>