hw/core/qdev-properties-system: Allow the 'slew' policy only on x86

hw/core/qdev-properties-system: Allow the 'slew' policy only on x86

The 'slew' tick policy is currently enforced to be only available on
x86 via some "#ifdef TARGET_I386" statements in mc146818rtc.c. We
want to get rid of those #ifdefs, so we need a different way of
checking whether the policy is allowed or not. Using the setter
function in hw/core/qdev-properties-system.c seems to be a good
place, so let's add a check here.

Suggested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230110095351.611724-3-thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>