nfc: trf7970a: mark OF related data as maybe unused

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

  drivers/nfc/trf7970a.c:2232:34: error: ‘trf7970a_of_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/nfc/trf7970a.c

@@ -2229,7 +2229,7 @@ static const struct dev_pm_ops trf7970a_pm_ops = {
 			   trf7970a_pm_runtime_resume, NULL)
 };
 
-static const struct of_device_id trf7970a_of_match[] = {
+static const struct of_device_id trf7970a_of_match[] __maybe_unused = {
 	{.compatible = "ti,trf7970a",},
 	{},
 };