thermal: tegra: mark PM functions __maybe_unused

After the PM support has been added to this driver, we get
a harmless warning when that support is disabled at compile
time:

drivers/thermal/tegra/soctherm.c:641:12: error: 'soctherm_resume' defined but not used [-Werror=unused-function]
 static int soctherm_resume(struct device *dev)

This marks the two PM functions as __maybe_unused to shut up
the warning. This is preferred over adding an #ifdef around
them, as it is harder to get wrong, and provides better
compile-time coverage.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: a134b4143b65 ("thermal: tegra: add PM support")
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>