syscon: Add syscon registers

These provides some info about the SoC (though it's still somewhat
incomplete and needs more work, see comments).

There's also a control register for selecting DRAM vs. BRAM at 0
(and for soft-resetting the SoC but that isn't wired up yet).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

microwatt.core

@@ -46,6 +46,7 @@ filesets:
       - wishbone_bram_wrapper.vhdl
       - soc.vhdl
       - xics.vhdl
+      - syscon.vhdl
     file_type : vhdlSource-2008
 
   fpga: