MAINTAINERS: Update with Amlogic DRM bindings converted as YAML

The amlogic,meson-dw-hdmi.txt and amlogic,meson-vpu.txt has been
converted to YAML schemas, update MAINTAINERS to match them again.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Maxime Jourdan <mjourdan@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808085522.21950-4-narmstrong@baylibre.com