First, does the PSP's processor have a MMU? If not, then there is not much
point in porting Linux (or for that matter, anything else). A machine
without a MMU is pretty worthless under Linux except as a fixed-function
embedded system; to wit, mmap(2) will not work.
Next, see http://www.linux-mips.org/wiki/index.php/Porting - it's
excellent. I don't know if a PSP has an accessible serial port - if not,
then you are somewhat screwed (OK, so not that bad; I've done a port that
used graphical display from the beginning for all debugging purposes).
You've got to do three things:
1) use lines no longer than 80 chars in your mails ;)
2) get all docs on the PSP you can get - reverse engineering is fun but
only if you enjoy hacking for 14 hours a day,
3) take a look at some directory in arch/mips.
Of course, the hardest of them is (2) in my experience, unless Sony felt
generous at the time and released all PSP docs. My port was completely
reverse-engineered but I wouldn't advise to do that unless you are sure
you can spare the time and effort.
Oh, and one more thing: USE THE CVS LINUX-MIPS.ORG TREE! The kernel.org
tree is so out of sync that it's virtually worthless.