dt-bindings: clock: mediatek: add entry for Mali-450 node to refer

Just add binding for a required clock referenced by Mali-450 on MT7623
or MT2701 SoC.

Cc: devicetree@vger.kernel.org
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>