dt-bindings: power: bq27xxx: add bq78z100

Add bindings for TI BQ78Z100. An I2C interface gas gauge.
It provides a fully integrated safety protection
and authentication for 1 to 2-series cell Li-Ion and
Li-Polymer battery packs.

Signed-off-by: LI Qingwu <Qing-wu.Li@leica-geosystems.com.cn>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>