dt-bindings: clock: add clock for MT2712

Add new clock according to 3rd ECO design change.
It's the parent clock of audio clock mux.

Signed-off-by: Weiyi Lu <weiyi.lu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>