clk: tegra: Propagate clk_out_x rate to parent

Given that externx can only be used as a parent for clk_out_x, it makes
sense to propagate requests to make clk_out_x easier to handle.

Signed-off-by: Alex Frid <afrid@nvidia.com>
Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>