arm: finish VGIC initialisation explicitly

Since Linux 3.19-rc1 there is a new API to explicitly initialise
the in-kernel GIC emulation by a userland KVM device call.
Use that to tell the kernel we are finished with the GIC
initialisation, since the automatic GIC init will only be provided
as a legacy functionality in the future.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>