dt-bindings: documentation: add clock bindings information for Stratix10

Document that Stratix10 clock bindings, and add the clock header file. The
clock header is an enumeration of all the different clocks on the Stratix10
platform.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>