AArch{32,64}: use KVM_CREATE_DEVICE & co to instanciate the GIC

As of 3.14, KVM/arm supports the creation/configuration of the GIC through
a more generic device API, which is now the preferred way to do so.

Plumb the new API in, and allow the old code to be used as a fallback.

[Andre: Rename some functions on the way to differentiate between
creation and initialisation more clearly and fix error path.]

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>