x86: use CONFIG_X86_SMP instead of CONFIG_SMP

Impact: fix x86/Voyager boot

CONFIG_SMP is used for features which work on *all* x86 boxes.
CONFIG_X86_SMP is used for standard PC like x86 boxes (for things like
multi core and apics)

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>