regulator: mp8859: Mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/regulator/mp8859.c:132:34: error: ‘mp8859_dt_id’ defined but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230310214553.275450-3-krzysztof.kozlowski@linaro.org


Signed-off-by: Mark Brown <broonie@kernel.org>