rtc: meson: mark PM functions as __maybe_unused

rtc: meson: mark PM functions as __maybe_unused

The meson_vrtc_set_wakeup_time() function is only used by
the PM functions and causes a warning when they are disabled:

drivers/rtc/rtc-meson-vrtc.c:32:13: error: unused function 'meson_vrtc_set_wakeup_time' [-Werror,-Wunused-function]

Remove the #ifdef around the callers and add a __maybe_unused
annotation as a more reliable way to avoid these warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20190906152438.1533833-1-arnd@arndb.de
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>