juno/thermal: Add notifications for SYSTOP

Power state transitions for SYSTOP need to be
received to stop the alarm when SYSTOP is down and
restart the alarm when SYSTOP is back on.
This will prevent the thermal protection attempting
to access resources not available when SYSTOP is down.

Change-Id: I2bfdb39f4f0679590f8abb3f147dab2ac5618cb3
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>