Correct macobio functions to only read and write one byte of data instead
of four. This fix a kernel panic on PowerMac G5 present since r1.22 of
i2s.c, issue reported by Chris Jackman, thanks!

Unify various gpio access using methods that take an offset relative to the
mac-io bus base address. Needed by upcoming dfs(4) support.

On some Apple machines, the openfirmware returns a gpio offset relative to
the gpio controller (generaly at 0x50). These offsets should be corrected
to be relative to the bus base address.

looks fine to kettenis@, ok miod@