media: MAINTAINERS: update drm/stm drm/sti and cec/sti maintainers

Add Alain as sti maintainer for both drm/sti & cec/sti.
Add Raphaël as stm maintainer for drm/stm.

Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Reviewed-by: Alain Volmat <alain.volmat@foss.st.com>
Reviewed-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>