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.
Allow 8 bit volume for YM2610 ADPCM-B
Remove sample mode macro in YM2610, it's always enabled and seperated channels.
TODO: ADPCM-B is still not implemented, FM Channel 2 is silenced in extended channel 3 configuration