When CONFIG_SUSPEND is not enabled, none of the system PM methods are
used, so do not compile them in.

Thanks to Charles Manning for reporting the problem and proposing
an initial patch.
Reported-by: NCharles Manning <manningc2@actrix.gen.nz>
Signed-off-by: NKevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>