dt-bindings: arm: imx: scu: drop deprecated legacy clock binding

The legacy clock binding are not maintained anymore. It has only
a very preliminary supported clocks during initial upstream and
meaningless for users. So drop it from binding doc now.

Cc: devicetree@vger.kernel.org
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>