arm-virt: refactor gpios creation

No functional change. Just refactor code to better
support secure and normal world gpios.

Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>