dt-bindings: power: supply: sbs-battery: Fix dtbs_check

Existing in-tree users make use of '"ti,bq20z45", "sbs,sbs-battery"', so
add it to the list of known compatible strings.

Also add missing support for the generic 'power-supplies' property by
importing power-supply.yaml and setting unevaluatedProperties instead
of additionalProperties.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>