Added const.sv with generic definitions

src/const.sv

@@ -0,0 +1,6 @@
+// Processor architecture
+`define OISC
+ 
+// Number of 16bit cells in ram 
+`define RAM_SIZE 4096  
+

src/top.sv

@@ -4,8 +4,7 @@
  * and SDRAM controller. 
 */
 
-// Compile OISC?
-`define OISC
+`include "const.sv"
 
 `ifdef OISC
 `include "oisc/cpu.sv"