drivers: rtc: driver not marked experimental in Kconfig

[ghost]

Describe the bug

The RTC driver is marked experimental in the docs but the corresponding EXPERIMENTAL flag in Kconfig is missing.

To Reproduce

Build:

1. Build an app with CONFIG_RTC=y and CONFIG_WARN_EXPERIMENTAL=y.
2. No warning is issued on the console.

Configuration:

1. Use your favorite Kconfig frontend, look up the RTC driver
2. No hint is given that the RTC API is experimental

Expected behavior

The RTC API should issue build warnings and clearly mention in Kconfig help/description that it is an experimental API so that users know what they are dealing with.

Maybe this should be backported to 3.4 where most users will see the API for the first time?

See linked PR.

Impact

Annoyed users when breaking the API while it's not stable.

Logs and console output

see above

Environment (please complete the following information):

any

Additional context

see #19030 (comment)

[ghost]

Closing as "won't fix" due to maintainer decision, see #59902 (comment)