MIPS: SGI-IP27: restructure ioc3 register access

Break up the big ioc3 register struct into functional pieces to
make use in sub-function drivers more straightforward. And while
doing that get rid of all volatile access by using readX/writeX.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>