dt-bindings: power: supply: Document Mitsumi MM8013 fuel gauge

The Mitsumi MM8013 is an I2C fuel gauge for Li-Ion cells. The partial
datasheet is available at [1]. Add bindings for this chip.

[1] https://www.mitsumi.co.jp/latest-M/Catalog/pdf/battery_mm_8013_e.pdf

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20230621-topic-mm8013-v4-2-975aecd173ed@linaro.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>