Adds switch --arm-v8.0 to FVP command line to enable ARM v8.0
architecture only.

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (FVP)
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>