clk: tegra: Add Tegra OSC to clock lookup

OSC is one of the parent for Tegra PMC clocks clk_out_1, clk_out_2,
and clk_out_3.

This patch adds Tegra OSC to clock lookup.

Tested-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>