Since clocks are not QOM objects, replace PROP_PTR of clocks with
setters methods.
Move/adapt the existing TODO comment about a clock framework.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>