MIPS: SGI-IP22: set PHYS_OFFSET to memory start

IP22 started at physical 0x08000000. To avoid wasting memory for
page structs set PHYS_OFFSET.

Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
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