ARM: v6k: select generic atomic64 code according to V6 variants

If CONFIG_CPU_V6 is enabled, avoid using the double-word exclusive
instructions in the kernel's atomic implementations as these are not
supported.  Fall back to the generic spinlock code instead.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Will Deacon <will.deacon@arm.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>