Did anyone got the documents on PSP device yet?
Other than the official Sony SDK documentation, as far as I know there is no low-level documentation on the PSP hardware. That includes some unfortunately complex subsystems, like the Media Engine or whatever it's called, which handles things like audio and video decompression. It would kind of suck to have an emulated PSP that can't play in-game videos or audio.
Who will be working on the source for MAME?
For now, nobody. I'm very interested in emulating the PSP, but since none of the other PSP emulator authors seem to be interested in emulating it using anything other than HLE bullshit, there isn't a whole lot of documentation or
low-level knowledge to go around. And I'm just not that confident in my MIPS abilities to be able to reverse-engineer the information that's necessary.
I wonder what is the hardest. Games files or the games ISO.
In fact, I'm not sure it's either of those: The real hard part is the fact that (just as far as I know) there aren't even any firmware dumps, or dumps of initial bootloader. Without either of those, it would be pretty hard to emulate the PSP properly in MAME.
If you know of any sites that actually have a dumped PSP BIOS or other firmware dumps, I think we'd all love to know about them. At the very least they could be made into a skeleton driver.