Now that the RTC_CHANGE event is no longer target-specific,
we can move the pl031 back to a compile-once source file
rather than a compile-per-target one.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20220221192123.749970-4-peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>