Add reverse engineering reference of PGM BIOS in info

[cam900]

https://github.com/wickerwaka/PGM-BIOS

Someone reverse engineered PGM BIOS, and that info has RAM area reserved for BIOS.

pgm_p02s_raw* is for 68000 BIOS data.

[RetroHQ]

Not sure if this is only valid when the BIOS is actually running in the foreground or whether it's used during game use. Need to have a look at what memory regions games use to see if these are avoided. If these memory areas are no-go for games it would need documenting, otherwise I'd say this is out of the scope of this reference.