dt-bindings: power: supply: Add DT schema for richtek,rt5033-battery

The RT5033 PMIC provides a simple fuel gauge via I2C.
Add a DT schema to describe how to set it up in the device tree.

Note that although RT5033 is a MFD with lots of functionality
(also charger, regulator, LEDs, ...) the fuel gauge has a separate
I2C bus and is not part of the MFD.

Cc: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>