clk: tegra: Add Tegra210 device tree binding

Add a header file that defines the clock numbers for Tegra210. It is
meant to be included by device trees so that they can refer to the
clocks by symbolic name instead of numeric value.

Also add the device tree binding documentation which is largely the
same as for earlier generations of Tegra.

Extracted from a larger patch by Rhyland Klein <rklein@nvidia.com>.

Signed-off-by: Thierry Reding <treding@nvidia.com>