MIPS: SGI-IP27: reduce ARC usage to a minimum

IP27 uses ARC prom only for parsing prom arguments and has a hack
for IP27 to make the ARC code behave. By introducing config symbol
ARC_CMDLINE_ONLY IP27 only drags in ARC cmdline parsing and does
everything else in IP27 specific code.

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