ARMv7: Check whether the SMP/nAMP mode was already enabled

If running in non-secure mode, enabling this register will fault.

Signed-off-by: Tony Thompson <Anthony.Thompson@arm.com>
Acked-by: Srinidhi Kasagar <srinidhikasagar@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>