rtc: sh: mark PM functions as unused

The sh_rtc_set_irq_wake() function is only called from the suspend/resume handlers
that may be hidden, causing a harmless warning:

drivers/rtc/rtc-sh.c:724:13: error: 'sh_rtc_set_irq_wake' defined but not used [-Werror=unused-function]
 static void sh_rtc_set_irq_wake(struct device *dev, int enabled)

The most reliable way to avoid the warning is to remove the existing #ifdef
and mark the two functions as __maybe_unused so the compiler can silently
drop all three when there is no reference.

Fixes: dab5aec6 ("rtc: sh: add support for rza series")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>