dt-bindings: power: supply: pm8941-coincell: Don't require charging properties

It's fine for these properties to be absent, as the driver doesn't fail
without them and functions with settings inherited from the reset/previous
stage bootloader state.

Fixes: 6c463222 ("dt-bindings: power: supply: pm8941-coincell: Convert to DT schema format")
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>