power: qpnp-fg: report battery resistance

Add support to report the battery resistance which
is a half float point encoded value as the following
power supply property:

  * POWER_SUPPLY_PROP_RESISTANCE

Change-Id: I6630c276579d7166a64c665cdfde99fc1d0d247e
Signed-off-by: David Keitel <dkeitel@codeaurora.org>