I know it's not complete but the wikipedia page for MIPS contains a few of the binary values of each instruction, can't really use hex to describe the instruction set well as it doesn't use 4-bit aligned values for each field (i.e 32 registers is 5 bits).
http://en.wikipedia.org/wiki/MIPS_architectureFor the rest you'd be better asking on the psp section of the ps2dev forums (or irc) as the likes of hlide and a few others on there are more likely to know about this.
Edit: These docs may be useful to you, dunno quite how much MIPS32R2 differs from classic mips though, I suspect that the basics like add, mul, bitwise, etc are still the same.
(classic mips)
http://www.d.umn.edu/~gshute/spimsal/talref.html(mips32)
http://www.math.unipd.it/~sperduti/ARCHITETTURE-1/mips32.pdf