YM2610(B): AY unification

as of now the SSG part of these chips is handled by a DivPlatformAY8910
within the DivPlatformYM2610.
this means less code duplication and therefore prepares for
OPN/OPNA support.

src/engine/platform/ay.h

@@ -87,6 +87,7 @@ class DivPlatformAY8910: public DivDispatch {
     void* getChanState(int chan);
     unsigned char* getRegisterPool();
     int getRegisterPoolSize();
+    void flushWrites();
     void reset();
     void forceIns();
     void tick();