[POWERPC] Fix drivers/macintosh/mediabay.c when !CONFIG_ADB_PMU

When building drivers/macintosh/mediabay.c if CONFIG_ADB_PMU isn't
defined we get:

drivers/built-in.o: In function `media_bay_step':
mediabay.c:(.text+0x92b84): undefined reference to `pmu_suspend'
mediabay.c:(.text+0x92c08): undefined reference to `pmu_resume'

Create empty place holders in that scenario.
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/linux/pmu.h

@@ -147,8 +147,15 @@ extern void pmu_wait_complete(struct adb_request *req);
 /* For use before switching interrupts off for a long time;
  * warning: not stackable
  */
+#if defined(CONFIG_ADB_PMU)
 extern void pmu_suspend(void);
 extern void pmu_resume(void);
+#else
+static inline void pmu_suspend(void)
+{}
+static inline void pmu_resume(void)
+{}
+#endif
 
 extern void pmu_enable_irled(int on);