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Clock and reset generation module

This module should be used as the main reset and clock generation module. It 
should have the asynchronous resets and unbuffered clock lines coming in, and 
depending on defines in the main design file, generate the appropriate clocks 
and clock-synchronous resets.

Currently Actel and Xilinx technlogies are supported. It is unclear if the way 
it is handling clock-configurations depending on boards is ideal, potentially 
it could become very cluttered if further board support is added and this would 
need to be looked at.

The technology-dependent modules (PLLs, buffers) instantiated here should be 
located under the backend/vendor/rtl paths.

As mentioned previously, it is unclear if this is the best way to manage the
task of clock and reset generation, particularly if further specialised clock
domains wish to be added, however there is some benefit in that it is all 
managed in one place and with the strict rules that all clocks and resets come 
to this module to be generated.