x86: silence mmconfig printk

There's so much broken mmconfig hardware/bios'es out there,
that classing this as an error seems a little extreme.
Lower its priority to KERN_INFO so that it isn't so noisy
when booting with 'quiet'

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>