vexpress: Check TC2 firmware support before defaulting to nonsec booting

The firmware on TC2 needs to be configured appropriately before booting
in nonsec mode will work as expected, so test for this and fall back to
sec mode if required.

Signed-off-by: Jon Medhurst <tixy@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>