Move SV interface files into a common location. Add license info (#20)

hdl-src/apb3_intf.sv

@@ -0,0 +1,40 @@
+interface apb3_intf #(
+    parameter DATA_WIDTH = 32,
+    parameter ADDR_WIDTH = 32
+);
+    // Command
+    logic PSEL;
+    logic PENABLE;
+    logic PWRITE;
+    logic [ADDR_WIDTH-1:0] PADDR;
+    logic [DATA_WIDTH-1:0] PWDATA;
+
+    // Response
+    logic [DATA_WIDTH-1:0] PRDATA;
+    logic PREADY;
+    logic PSLVERR;
+
+    modport master (
+        output PSEL,
+        output PENABLE,
+        output PWRITE,
+        output PADDR,
+        output PWDATA,
+
+        input PRDATA,
+        input PREADY,
+        input PSLVERR
+    );
+
+    modport slave (
+        input PSEL,
+        input PENABLE,
+        input PWRITE,
+        input PADDR,
+        input PWDATA,
+
+        output PRDATA,
+        output PREADY,
+        output PSLVERR
+    );
+endinterface