media: platform: stm32: defer probe for auxiliary clock

Change stm32-cec driver to defer probe when auxiliary clock
"hdmi-cec" is registered in the system not has not been probed yet.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>