MIPS: SGI-IP27: use asm/sn/agent.h for including HUB related stuff

By including agent.h where hub related defines/structs are needed, we
have only one place to select, which agent chip (HUB or BEDROCK) is used.

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