dt-bindings: cpufreq: tegra124: remove vdd-cpu-supply from required properties

The Tegra124 cpufreq driver works only with DFLL clock, which is a
hardware-based frequency/voltage controller. The driver doesn't need to
control the regulator itself. Hence remove that.

Cc: devicetree@vger.kernel.org
Signed-off-by: Joseph Lo <josephl@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>