ARM: mxs: stub out mxs_pm_init for !CONFIG_PM

When building a kernel without CONFIG_PM, we get a link
error from referencing mxs_pm_init in the machine
descriptor. This defines a macro to NULL for that case.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/mach-mxs/pm.h

@@ -9,6 +9,10 @@
 #ifndef __ARCH_MXS_PM_H
 #define __ARCH_MXS_PM_H
 
+#ifdef CONFIG_PM
 void mxs_pm_init(void);
+#else
+#define mxs_pm_init NULL
+#endif
 
 #endif