dt-bindings: clk: intel: Add bindings document & header file for CGU

Clock generation unit(CGU) is a clock controller IP of Intel's Lightning
Mountain(LGM) SoC. Add DT bindings include file and document for CGU clock
controller driver of LGM.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Link: https://lkml.kernel.org/r/8dce2be13195aab20c6b11fca6af0fffe22d5241.1587102634.git.rahul.tanwar@linux.intel.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>