dt-bindings: imx-cpufreq-dt: Document opp-supported-hw usage

The interpretation of opp-supported-hw bits for imx-cpufreq-dt driver is
not very obvious so attempt to explain it.

There is no OF compat string associated.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>