dt-bindings: power: Extend battery bindings with chemistry

This adds a battery-chemistry property and bindings for the different
"technologies" that are used in Linux. More types can be added.

This is needed to convert the custom ST-Ericsson AB8500 battery
properties over to the generic battery bindings.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>