power: qpnp-fg: add support to detect empty battery based on vbatt-low irq

On some platforms, the empty soc interrupt fires randomly and it
cannot be trusted to report battery empty condition. Use Vbatt
low interrupt along with delta soc interrupt to determine whether
battery is really empty.

This option of using vbatt-low interrupt to detect empty battery
is supported via "qcom,fg-use-vbat-low-empty-soc" property.

CRs-Fixed: 977670
Change-Id: I757a1f3c7c66d33d88495348f5dce40cb05cfa64
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>