MAINTAINERS: Extend ARM/Mediatek SoC support section

Catch mt7623 and arm64 dts subdirectory.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>