MIPS: SGI-IP27: Use union instead of typedef

The upcoming header files for SGI-IP35 shouldn't add new typedef. To
be able to share ip27-hubio.c we need to access register layouts no
longer via a typedef.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org