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

drivers/regulator/mp8859.c

@@ -129,7 +129,7 @@ static int mp8859_i2c_probe(struct i2c_client *i2c)
 	return 0;
 }
 
-static const struct of_device_id mp8859_dt_id[] = {
+static const struct of_device_id mp8859_dt_id[] __maybe_unused = {
 	{.compatible =  "mps,mp8859"},
 	{},
 };