dt-bindings: clock: imx8m: Integrate duplicated i.MX 8M schemas

The clock controller schemas for i.MX 8M Mini, 8M Nano, 8M Plus and 8M
Quad are basically the same.  The only minor difference appears on 8M
Quad which needs one more clock.

There is no point to have four schemas for almost the same binding.  Any
fixes or changes would have to be duplicated four times.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>