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

fvp: rename fvp_* files to plat_* ...

The FVP platform has a few filenames that begin with fvp_.  These are
renamed to plat_ to make it easier to use the FVP port as a template.

Change-Id: I601e6256d5ef3bae81a2e1f5df6de56db5b27069
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>