juno/xrp7724: Move shutdown & reset to full async handling

This patch enables the system_mode_api to handle deferred requests.

With this change, the constraint for the caller to run on its thread
to make correct use of the above API is removed.

Change-Id: I16b07e1e5669af7fdc380fcaecf1e1236e49405e
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>