Make the system poweroff and RTC wakeup part of the driver optional,
under a boolean config option. Move the dependency to RTC_CLASS to this
new option.

This makes the turris-omnia-mcu driver available for compilation even if
RTC_CLASS is disabled.

Fixes: ed46f1f7 ("platform: cznic: turris-omnia-mcu: fix Kconfig dependencies")
Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20240719085756.30598-4-kabel@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>