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>