mfd: atmel-smc: Mark OF related data as maybe unused

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

  drivers/mfd/atmel-smc.c:326:34: error: ‘atmel_smc_ids’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230311111629.251830-1-krzysztof.kozlowski@linaro.org

drivers/mfd/atmel-smc.c

@@ -323,7 +323,7 @@ static const struct atmel_hsmc_reg_layout sama5d2_reg_layout = {
 	.timing_regs_offset = 0x700,
 };
 
-static const struct of_device_id atmel_smc_ids[] = {
+static const struct of_device_id atmel_smc_ids[] __maybe_unused = {
 	{ .compatible = "atmel,at91sam9260-smc", .data = NULL },
 	{ .compatible = "atmel,sama5d3-smc", .data = &sama5d3_reg_layout },
 	{ .compatible = "atmel,sama5d2-smc", .data = &sama5d2_reg_layout },