dt-bindings: soc: tegra-pmc: Add Tegra PMC clock bindings

Tegra PMC has 3 clocks clk_out_1, clk_out_2, and clk_out_3.

This patch documents PMC clock bindings and adds a header defining
Tegra PMC clock ids.

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