PM / OPP: Fix build when CONFIG_PM_OPP is not set

Commit 03ca370f

 (PM / OPP: Add
OPP availability change notifier) does not compile if CONFIG_PM_OPP
is not set:

arch/arm/plat-omap/omap-pm-noop.o: In function `opp_get_notifier':
include/linux/opp.h:103: multiple definition of `opp_get_notifier'
include/linux/opp.h:103: first defined here

Also fix incorrect comment.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>