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: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Reviewed-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/20220113171921.17466-3-philippe.cornu@foss.st.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>