fvp: clear a pending cluster power off request

The last CPU in a cluster is responsible for issuing the cluster power
down request to the FVP power controller. If another CPU in this
cluster wakes up before the last CPU enters WFI then the cluster power
down request remains pending. If this request is not cancelled and the
newly woken up CPU enters a simple WFI later, the power controller
powers the cluster down. This leads to unpredictable behaviour.

This patch fixes this issue by ensuring that the first CPU to wake up
in a cluster writes its MPIDR to the power controller's PPONR. This
cancels any pending cluster power down request.

Change-Id: I7e787adfd6c9a0bd7308390e3309d46f35c01086