arm: juno: Use PSCI based reset

So far the Juno board wasn't implementing reset. Let's just use the
already existing PSCI_RESET based method to avoid any extra code.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>