pl011: Subscribe to pre-shutdown notifications

To correctly perform the shutdown sequence the pl011
needs to be notified such that it can transparently disable
any read/write.

Change-Id: Id90f2b8b42be8b8b424b84ab05202c7ad6b978f7
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>