ARM: tegra20: create a DT header defining CLK IDs

ARM: tegra20: create a DT header defining CLK IDs

Create a header file to define the clock IDs used by the Tegra20 clock
binding. Remove the list of definitions from the binding documentation,
and refer the reader to the header file.

This will allow the same header to be used by both device tree files,
and drivers implementing this binding, which guarantees that the two
stay in sync. This also makes device trees more readable by using names
instead of magic numbers.

Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
[swarren, add header to clock/ instead of clk/ to match binding location]
Signed-off-by: Stephen Warren <swarren@nvidia.com>