xen/arm: switch to dynamic allocation of vgic rank

xen/arm: switch to dynamic allocation of vgic rank

vgic_irq_rank structure contains gic specific data elements.
Move this out of domain.h to new vgic header file vgic.h
Allocate memory dynamically in vgic driver.

This patch reduces the size of domain struct and helps to
keep domain struct within PAGE_SIZE when future GIC hw versions
are added

Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>