power: vm-bms: Update fifo length at low SOC

The VBAT drops very fast at lower SOCs. Add a logic to
reduce the fifo length at lower SOC to allow more
frequent fifo update events. Add a DT property to specify
the fifo-length at low SOC. The fifo-length is not
updated if this property is not set.

While at it, log the configured FIFO length for S1 and S2.

Change-Id: I9a09dada26f0b5f98c03c8b4aaf997846f22d1dd
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>