spi: sh-msiof: Mark OF related data as maybe unused

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

  drivers/spi/spi-sh-msiof.c:1076:34: error: ‘sh_msiof_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230310222857.315629-14-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-sh-msiof.c

@@ -1073,7 +1073,7 @@ static const struct sh_msiof_chipdata rcar_gen3_data = {
 	.min_div_pow = 1,
 };
 
-static const struct of_device_id sh_msiof_match[] = {
+static const struct of_device_id sh_msiof_match[] __maybe_unused = {
 	{ .compatible = "renesas,sh-mobile-msiof", .data = &sh_data },
 	{ .compatible = "renesas,msiof-r8a7743",   .data = &rcar_gen2_data },
 	{ .compatible = "renesas,msiof-r8a7745",   .data = &rcar_gen2_data },