arm/gic-v2: Add ACPI boot support for GICv2

arm/gic-v2: Add ACPI boot support for GICv2

ACPI on Xen hypervisor uses MADT table for proper GIC initialization.
First get the GIC version from GIC Distributor. Then parse GIC related
subtables, collect CPU interface and distributor addresses and call
driver initialization function (which is hardware abstraction agnostic).
In a similar way, FDT initialize GICv2.

Signed-off-by: Parth Dixit <parth.dixit@linaro.org>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>