spi: bcm-qspi: Mark OF related data as maybe unused

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

  drivers/spi/spi-bcm-qspi.c:1460:34: error: ‘bcm_qspi_of_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230310222857.315629-13-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm-qspi.c

@@ -1457,7 +1457,7 @@ static const struct bcm_qspi_data bcm_qspi_spcr3_data = {
 	.has_spcr3_sysclk = true,
 };
 
-static const struct of_device_id bcm_qspi_of_match[] = {
+static const struct of_device_id bcm_qspi_of_match[] __maybe_unused = {
 	{
 		.compatible = "brcm,spi-bcm7445-qspi",
 		.data = &bcm_qspi_rev_data,