I have put an initial version of raspberryPi hardware RNG driver on sources. Supposedly bcm2835 uses a reverse bias transistor as a noise source (though I couldn't find anything a definitive source for this). FWIW, I ran the output through rngtest (does FIPS 140-2 tests) and the failure rate is about the same as FreeBSD's (about 1 out of 1000). rngtest tests at a minimum 2*10^6 bits and plan9's stock /dev/random seems far too slow so I didn't bother testing it.
Just copy /n/sources/contrib/bakul/random.c to sys/src/9/bcm and rebuild 9pi. Arguably this should be a new device, /dev/hrng or something, but plan9's /dev/random code is very simple compared to the elaborate versions on *BSD's. I have kept this one simple too. If you are concerned about the quality of RNG or paranoid about bcm2835's unpublic algorithm, feel free to complexify!
