Oh lord, why would anyone want soundfont samples of FM synthesizers?
For Genesis/Mega Drive your best bet is to get the VOPM VSTi (which is free). It's not modeled on YM2612 specifically, but it is modeled on YM2151 which is pretty close. It should be able to do the job.
If you want to go for like a SNES sound, just use some small, short, 32khz 16-bit samples in any sampler that has gaussian interpolation. Or in a tracker. Or for super authenticity, you can even track your stuff in XM and then use eKid's XMSNES.
Looks like XMSNES isn't going to be developed any more though . Maybe there's some alternative out there somewhere...