dt-bindings: power: supply: pm8941-coincell: Add PM8998 compatible

Add a specific compatible for the coincell charger present on PM8998.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>