ARM: Avoid building unsafe kernels on OMAP2 and MX3

OMAP2 (armv6) and MX3 turn off support for the V6K instructions, which
when they include support for SMP kernels means that the resulting
kernel is unsafe on SMP and can result in corrupted filesystems as we
end up using unsafe bitops.

Re-enable the use of V6K instructions on such kernels, and let such
kernels running on V6 CPUs eat undefined instruction faults which will
be much safer than filesystem corruption.  Next merge window we can fix
this properly (as it requires a much bigger set of changes.)

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>