drivers: rtc: Create utility function for time validation

RTC drivers should validate the `struct rtc_time`'s contents against the
provided `mask`. Promote this common code to a new rtc_utils file and
modify existing drivers to use this functionality. Extend the test
coverage to include verifying this behaviour.

This is groundwork ahead of adding support for the RP2040's (as used in
the Raspberry Pi Pico) RTC and alarm.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>