systems: add atlys board port

OpenRISC system for Digilent Atlys development board.
Basically lifted 'as is' from the orpsocv2 port at:
git://git.openrisc.net/stefan/orpsoc

TODO:
 - Rewrite dvi_gen (currently a hacked up Xilinx whitepaper)
 - Clean up the Xilinx MIG memory controller wrapper
   (and possibly generate the MIG with coregen on fly)
 - General cleanup and de-orpsocv2-fication