OMAP2: PM: fix compile error when !CONFIG_SUSPEND

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: Charles Manning <manningc2@actrix.gen.nz>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>