ARM: Activate FIQs to avoid __irq_startup warnings

There is a new test in __irq_startup that the IRQ is activated, which
hasn't been the case for FIQs since they bypass some of the usual setup.

Augment enable_fiq to include a call to irq_activate to avoid the
warning.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>