nvmem: stm32-romem: mark OF related data as maybe unused

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

  drivers/nvmem/stm32-romem.c:271:34: error: ‘stm32_romem_of_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230404172148.82422-27-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/nvmem/stm32-romem.c

@@ -268,7 +268,7 @@ static const struct stm32_romem_cfg stm32mp13_bsec_cfg = {
 	.ta = true,
 };
 
-static const struct of_device_id stm32_romem_of_match[] = {
+static const struct of_device_id stm32_romem_of_match[] __maybe_unused = {
 	{ .compatible = "st,stm32f4-otp", }, {
 		.compatible = "st,stm32mp15-bsec",
 		.data = (void *)&stm32mp15_bsec_cfg,