power: qpnp-fg: configure irq_volt_empty in the fuel gauge

The empty soc interrupt in the fuel gauge is the last line of defense
for the fuel gauge to report 0% before the phone runs out of battery
power. Allow the threshold at which this interrupt triggers at to be
configured via the device tree in the fuel gauge driver as
qcom,irq-volt-empty-mv.

CRs-Fixed: 758265
Change-Id: I6f6fa07e3cca1119d7ff6e6cab9e20100e0eea80
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>