xen/arm: split vgic driver into generic and vgic-v2 driver

xen/arm: split vgic driver into generic and vgic-v2 driver

Existing vGIC driver has both generic code and hw specific
code. Segregate vGIC low level driver into vgic-v2.c and
keep generic code in existing vgic.c file.

Some static generic functions in vgic.c is made as non-static
so that these generic functions can be used in vGIC v2 driver.

vGIC v2 driver registers required callbacks to generic vGIC
driver. This helps to plug in next version of vGIC drivers
like vGIC v3. These callbacks are registered per domain

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