ARM: v6k: only allow SMP if we have v6k or v7 CPU

SMP extensions are only supported on ARMv6k or ARMv7 architectures, so
only offer the option if we're building for such an architecture.

Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>