dt-bindings: mfd: dlg,da9063: Document voltage monitoring

Document that the da9063 only provides under- *and* over-voltage
monitoring in one, and therefore requires both to be configured with the
same severity and value. Add an example for clarification.

Signed-off-by: Benjamin Bara <benjamin.bara@skidata.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230403-da9063-disable-unused-v3-3-cc4dc698864c@skidata.com