mailbox: tegra-hsp: mark suspend function as __maybe_unused

We get a warning about this being unused when CONFIG_PM is disabled:

drivers/mailbox/tegra-hsp.c:782:12: error: 'tegra_hsp_resume' defined but not used [-Werror=unused-function]

Fixes: 9a63f0f4 ("mailbox: tegra-hsp: Add suspend/resume support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/tegra-hsp.c

@@ -779,7 +779,7 @@ static int tegra_hsp_probe(struct platform_device *pdev)
 	return 0;
 }
 
-static int tegra_hsp_resume(struct device *dev)
+static int __maybe_unused tegra_hsp_resume(struct device *dev)
 {
 	struct tegra_hsp *hsp = dev_get_drvdata(dev);
 	unsigned int i;