juno/ppu: Add polling for core power state in system suspend

For system suspend, once the last standing core has entered state OFF,
the power state transition must be reported to power_domain. Since
there is no interrupt capability in Juno, this is achieved instead by
polling the core policy status via a timer alarm.

This patch adds such polling functionality and module configuration.

Change-Id: I016bc5bd844b0d7b7c9e0cc4bc450168e1d00d91
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>