ARM: tegra: pm: Add tegra_cpu_do_idle() prototype

When building with W=1 this function is flagged as not having a
prototype defined. Add the prototype to avoid a build warning.
Signed-off-by: Thierry Reding <treding@nvidia.com>

arch/arm/mach-tegra/pm.h

@@ -36,7 +36,7 @@ void tegra30_sleep_core_init(void);
 
 void tegra_clear_cpu_in_lp2(void);
 bool tegra_set_cpu_in_lp2(void);
-
+int tegra_cpu_do_idle(void);
 void tegra_idle_lp2_last(void);
 extern void (*tegra_tear_down_cpu)(void);