[PowerPC] De-giant powermac_nvram, update documentation

[bdragon]

Summary

• Remove the giant lock requirement from powermac_nvram.
• Update manual pages to reflect current state.

(Currently tested on G4 powerbook)

[bdragon]

sys/dev/powermac_nvram/powermac_nvram.c

277 ↗

(On Diff #71657)

I'm not really a fan of the way this just ignores the requested position. If we were to honor uio_offset instead, we could avoid locking here.

I guess in practice this driver is exclusive access to start with in any case, as it stores a copy of the nvram in the sc while the device is open, so it doesn't *really* matter.

sys/dev/powermac_nvram/powermac_nvramvar.h

52 ↗

(On Diff #71657)

I originally tried this with a mutex, but it turns out that uiomove() can go to sleep.

[bdragon]

usr.sbin/nvram/nvram.8

101 ↗

(On Diff #71657)

There are a couple of other manpages that reference the eeprom tool, which is gone ever since the sparc64 removal.

[bcr]

OK from manpages.