drm/etnaviv: Make the "core" clock mandatory

It is always present. It was documented as mandatory prior to
commit 90aeca87

 ("dt-bindings: display: Convert etnaviv to
json-schema").

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>