rtc: omap: mark PM methods as __maybe_unused

Instead of using #ifdef guards around PM methods, let's annotate
them as __maybe_unused, as it provides better compile coverage.

Also drop empty stub for omap_rtc_runtime_resume().

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>