platform: cznic: turris-omnia-mcu: Add support for poweroff and wakeup

Add support for true board poweroff (MCU can disable all unnecessary
voltage regulators) and wakeup at a specified time, implemented via a
RTC driver so that the rtcwake utility can be used to configure it.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20240701113010.16447-5-kabel@kernel.org


Signed-off-by: Arnd Bergmann <arnd@arndb.de>