spi: Fix tegra20 build with CONFIG_PM=n

Without CONFIG_PM enabled, the SET_RUNTIME_PM_OPS() macro ends up being
empty, and the only use of tegra_slink_runtime_{resume,suspend} goes
away, resulting in

  drivers/spi/spi-tegra20-slink.c:1200:12: error: ‘tegra_slink_runtime_resume’ defined but not used [-Werror=unused-function]
   1200 | static int tegra_slink_runtime_resume(struct device *dev)
        |            ^~~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/spi/spi-tegra20-slink.c:1188:12: error: ‘tegra_slink_runtime_suspend’ defined but not used [-Werror=unused-function]
   1188 | static int tegra_slink_runtime_suspend(struct device *dev)
        |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~

mark the functions __maybe_unused to make the build happy.

This hits the alpha allmodconfig build (and others).
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi-tegra20-slink.c

@@ -1185,7 +1185,7 @@ static int tegra_slink_resume(struct device *dev)
 }
 #endif
 
-static int tegra_slink_runtime_suspend(struct device *dev)
+static int __maybe_unused tegra_slink_runtime_suspend(struct device *dev)
 {
 	struct spi_master *master = dev_get_drvdata(dev);
 	struct tegra_slink_data *tspi = spi_master_get_devdata(master);
@@ -1197,7 +1197,7 @@ static int tegra_slink_runtime_suspend(struct device *dev)
 	return 0;
 }
 
-static int tegra_slink_runtime_resume(struct device *dev)
+static int __maybe_unused tegra_slink_runtime_resume(struct device *dev)
 {
 	struct spi_master *master = dev_get_drvdata(dev);
 	struct tegra_slink_data *tspi = spi_master_get_devdata(master);