[ARM] vexpress-a15: Use VGIC emulator instead GIC emulator where possible.

For using HW-assissted GICv2 emulation in a Guest we need the following:
1. Underlying Host must have GIC with virtualization extension
   and ARMv7 CPU with virtualization extensions
2. The Guest machine we are trying emulate must have both GIC
   CPU interface and GIC Distributor interface at 4K aligned
   Guest physical address

Currently, Realview-PB-A8 Guest and VExpress-A15 Guest on VExpress-A15
Host comply with above conditions hence, we use VGIC emulator for both
these Guest types on VExpress-A15 Host.

Signed-off-by: Anup Patel <anup@brainfault.org>