dt-bindings: power: supply: max17040: Add DT bindings for max17040 fuel gauge

Documentation of max17040 based fuel gauge characteristics.
For configure low level state of charge threshold alert signaled from
max17043/max17044 we add "maxim,alert-low-soc-level" property.

Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>