Major update to the FVP platform port to use the common platform code
in (include/)plat/arm/* and (include/)plat/common/*. This mainly
consists of removing duplicated code but also introduces some small
behavioural changes where there was unnecessary variation between the
FVP and Juno ports. See earlier commit titled `Add common ARM and CSS
platform code` for details.

Also add support for Foundation FVP version 9.1 during FVP config
setup to prevent a warning being emitted in the console.

Change-Id: I254ca854987642ce09d1b924c9fd410a6e13e3bc

Function declarations implicitly have external linkage so do not
need the extern keyword.

Change-Id: Ia0549786796d8bf5956487e8996450a0b3d79f32

Move the FVP power driver to a directory under the FVP platform
port as this is not a generically usable driver.

Change-Id: Ibc78bd88752eb3e3964336741488349ac345f4f0