[TF-A] Please provide option to build with GICv3 for qemu

Hello!

Please add a build option for selecting GICv3 instead of GICv2 for QEMU platform.

There is the following line in plat/qemu/qemu/platform.mk : QEMU_USE_GIC_DRIVER := QEMU_GICV2

It doesn't imply setting GICv3 externally by build system of other projects, like OP-TEE.

Best regards, Aleksey.