Add memory mapper software interface

Adds functions to read and write mappings, as well as enable and disable
the memory mapper.

This also moves increases the io space by 16 bytes.

hw/fpga/addr_decode.sv

@@ -11,12 +11,12 @@ module addr_decode(
 );
 
 assign rom_cs = addr >= 16'h8000;
-assign sdram_cs = addr < 16'h7ff0;
+assign sdram_cs = addr < 16'h7fe0;
+assign mm_cs1 = addr >= 16'h7fe0 && addr < 16'h7ff0;
 assign hex_cs = addr >= 16'h7ff0 && addr < 16'h7ff4;
 assign uart_cs = addr >= 16'h7ff4 && addr < 16'h7ff6;
 assign board_io_cs = addr == 16'h7ff6;
 assign mm_cs2 = addr == 16'h7ff7;
-assign mm_cs1 = addr >= 16'h7ff8 && addr < 16'h7ffc;
 assign irq_cs  = addr == 16'h7fff;
 
 endmodule