This patch changes the default driver for FVP platform from the deprecated
GICv3 legacy to the GICv3 only driver. This means that the default build of
Trusted Firmware will not be able boot Linux kernel with GICv2 FDT blob. The
user guide is also updated to reflect this change of default GIC driver for
FVP.

Change-Id: Id6fc8c1ac16ad633dabb3cd189b690415a047764