clk: tegra30: Use 300MHz for video decoder by default

The 600MHz is a too high clock rate for some SoC versions for the video
decoder hardware and this may cause stability issues. Use 300MHz for the
video decoder by default, which is supported by all hardware versions.

Fixes: ed1a2459

 ("clk: tegra: Add Tegra20/30 EMC clock implementation")
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>