mapper-related updates and fixes (#491)

* m116: Fix support for Huang-2 chip

* Provide 8K CHR ram if no CHR rom/ram is provided due to bad headers

* m15: Implement CHR write-protect

* Update Namco 106

- Cleanup save state struct
- Implement nametable mirroring
- fix sound issue (fceux)

Co-authored-by: negativeExponent <negativeExponent@users.noreply.github.com>

src/boards/71.c

@@ -46,6 +46,7 @@ static DECLFW(M71Write) {
 }
 
 static void M71Power(void) {
+	preg = 0;
 	mirr = 0;
 	Sync();
 	SetReadHandler(0x8000, 0xFFFF, CartBR);