dt-bindings: mfd: maxim,max8998: Convert to DT schema

Convert the bindings for Maxim MAX8998, National/TI LP3974 Power
Management IC to DT schema.  Adjust example to real DTS and make second
interrupt optional (like on s5pv210-aries.dtsi).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230911120135.37528-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Lee Jones <lee@kernel.org>