spi: pxa2xx: Mark OF related data as maybe unused

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

drivers/spi/spi-pxa2xx.c:1757:34: error: ‘pxa2xx_spi_of_match’ defined but not used [-Werror=unused-const-variable=]

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-pxa2xx.c

···
 MODULE_DEVICE_TABLE(acpi, pxa2xx_spi_acpi_match);
 #endif
 
-static const struct of_device_id pxa2xx_spi_of_match[] = {
+static const struct of_device_id pxa2xx_spi_of_match[] __maybe_unused = {
 	{ .compatible = "marvell,mmp2-ssp", .data = (void *)MMP2_SSP },
 	{}
 };