MAINTAINERS: dt: update etnaviv file reference

The etnaviv file was converted to json and renamed.

Update its reference accordingly.

Fixes: 90aeca87

 ("dt-bindings: display: Convert etnaviv to json-schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>