arm: always start from first_cpu when registering loader cpu reset callback

arm: always start from first_cpu when registering loader cpu reset callback

if arm_load_kernel() were passed non first_cpu, QEMU would end up
with partially set do_cpu_reset() callback leaving some CPUs without it.

Make sure that do_cpu_reset() is registered for all CPUs by enumerating
CPUs from first_cpu.

(In practice every board that we have was passing us the first CPU
as the boot CPU, either directly or indirectly, so this wasn't
causing incorrect behaviour.)

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: added a note that this isn't a behaviour change]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>