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

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

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/20200512181647.5071-4-dinguyen@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>