ARM: new VGIC: vgic-init: implement vgic_init

ARM: new VGIC: vgic-init: implement vgic_init

This patch allocates and initializes the data structures used to model
the vgic distributor and virtual cpu interfaces. At that stage the
number of IRQs and number of virtual CPUs is frozen.
Implement the various functions that the Xen arch code is expecting to
call during domain and VCPU setup to initialize the VGIC.
Their prototypes are already in existing header files.

This is based on Linux commit ad275b8bb1e6, written by Eric Auger.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>