Change the default driver to GICv3 in FVP

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
diff --git a/plat/arm/board/fvp/platform.mk b/plat/arm/board/fvp/platform.mk
index 60ebe65..eecb597 100644
--- a/plat/arm/board/fvp/platform.mk
+++ b/plat/arm/board/fvp/platform.mk
@@ -28,8 +28,8 @@
 # POSSIBILITY OF SUCH DAMAGE.
 #
 
-# Use the Legacy GICv3 driver on the FVP by default to maintain compatibility.
-FVP_USE_GIC_DRIVER	:= FVP_GICV3_LEGACY
+# Use the GICv3 driver on the FVP by default
+FVP_USE_GIC_DRIVER	:= FVP_GICV3
 
 # The FVP platform depends on this macro to build with correct GIC driver.
 $(eval $(call add_define,FVP_USE_GIC_DRIVER))