juno_system: Propagate returned status to the caller

The returned status from the driver is propagated to the
caller such that a deferred request information can be transferred
to the caller.

Change-Id: Icf951a11a39ff3a663854dffe174fecdfcdf65e5
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>