arm/dt: pm8226: add low-voltage-threshold property to BMS node

When the required property "qcom,low-voltage-threshold" was added to
the BMS driver, it was not included in the pm8226 devicetree. Add it
back in in order to allow BMS to probe.

Change-Id: If7575bdfba57d135e77d786c753a663791c493c8
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>