ARM: tegra: Remove second instance of uart clk

In Tegra30 clock file two clk objects were created for same UART clock.
Remove the duplicates and add clock aliases instead.

Update the Tegra20 clock file to make the names consistent.

Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>