[media] st-hva: update MAINTAINERS

Add entry for the HVA driver to the MAINTAINERS file.

Signed-off-by: Jean-Christophe Trotin <jean-christophe.trotin@st.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>