soc/tegra: pm: Make stubs usable for compile testing

The PM stubs need to depend on ARCH_TEGRA in order to be usable for
compile-testing of tegra-cpuidle driver. Add the dependency.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

include/soc/tegra/pm.h

@@ -17,7 +17,7 @@ enum tegra_suspend_mode {
 	TEGRA_SUSPEND_NOT_READY,
 };
 
-#if defined(CONFIG_PM_SLEEP) && defined(CONFIG_ARM)
+#if defined(CONFIG_PM_SLEEP) && defined(CONFIG_ARM) && defined(CONFIG_ARCH_TEGRA)
 enum tegra_suspend_mode
 tegra_pm_validate_suspend_mode(enum tegra_suspend_mode mode);