xen/arm: move GIC context data structure to gic driver

xen/arm: move GIC context data structure to gic driver

arch_domain in domain.h defines all the GIC registers
that needs to be saved/restored directly.

These GIC registers are GIC HW version specific. The number
of registers and size of registers varies with GIC version.
So move these registers to gic.h file and make a union of these
registers. This helps to define GIC HW version specific
structure to this union for later GIC versions.

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>