power: qpnp-qg: Update vbat_low handling in QG

On PMI632 2.0, the vbat_log irq can fire at both
edges. Make sure that the change to fifo-length
takes place on all vbat_low events.

While at it, add documentation for a 'qcom,qg-ext-sns'
DT property.

Change-Id: I32e1c42e58f739b277943ff07863249a1c40a4d1
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>