PM idle: remove global declaration of pm_idle

pm_idle appears in no generic Linux code,
it appears only in architecture-specific code.

Thus, pm_idle should not be declared in pm.h.

Architectures that  use an idle function pointer
should delcare one local to their architecture,
and/or use cpuidle.

Signed-off-by: Len Brown <len.brown@intel.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Cc: linux-pm@vger.kernel.org