include: drivers: rtc: counter: deprecate counter-based DS3231 driver

There is a "native" RTC driver for DS3231 now (maxim,ds3231-rtc, one of
the multiple functions implemented as MFD) so do all we can to
discourage the use of the legacy, counter-API based, driver.

Flag the compatible as deprecated.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>