ARM: at91: fixed unresolved symbol "at91_pm_set_standby" when built without CONFIG_PM

If CONFIG_PM is not defined, then arch/arm/mach-at91/pm.c is not
compiled in.  This patch creates an inline function that does nothing
if CONFIG_PM is not defined.
Signed-off-by: Brent Taylor <motobud@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/mach-at91/pm.h

@@ -16,7 +16,11 @@
 #include <mach/at91_ramc.h>
 #include <mach/at91rm9200_sdramc.h>
 
+#ifdef CONFIG_PM
 extern void at91_pm_set_standby(void (*at91_standby)(void));
+#else
+static inline void at91_pm_set_standby(void (*at91_standby)(void)) { }
+#endif
 
 /*
  * The AT91RM9200 goes into self-refresh mode with this command, and will