fvp: use DT file foundation-v8-gicv3-psci.dtb

The Device Tree configuration we need for the Foundation Platform is now
available upstream (v4.18) as foundation-v8-gicv3-psci.dts (it only lacks
the optee node and memory reservations). Let's use this new file instead
of a hacked version of foundation-v8.dts modified for PSCI and GiCv3.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>