[TESTS] Add generic timer for VExpress-A15 basic firmware.

We should be using generic timer in basic firmware for host
having virtualization extension and generic timers.

For now, we only have VExpress-A15 Guest and Virt-v8 Guest
with generic timer but we were not using generic timer for
basic firmware on VExpress-A15 Guest hence this patch.

Signed-off-by: Anup Patel <anup@brainfault.org>