ARM: Do not enable SWP emulation if CPU_V6 && CPU_V7

This option uses LDREXB/STREXB to emulate SWPB but these instructions
are not supported on all the ARMv6 processors.

Reported-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Leif Lindholm <Leif.Lindholm@arm.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>