[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>