x86: prevent /sys/firmware/sgi_uv from being created on non-uv systems

/sys/firmware/sgi_uv should only be created on uv systems.

Signed-off-by: Russ Anderson <rja@sgi.com>
LKML-Reference: <20090403222423.GA28546@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>