xen/arm: gic-v3: Add support of vGICv2 when available

xen/arm: gic-v3: Add support of vGICv2 when available

* Modify the GICv3 driver to recognize a such device. I wasn't able
  to find a register which tell if GICv2 is supported on GICv3. The only
  way to find it seems to check if the DT node provides GICC and GICV.

* Disable access to ICC_SRE_EL1 to guest using vGICv2

* The LR is slightly different for vGICv2. The interrupt is always
injected with group0.

* Add a comment explaining why Group1 is used for vGICv3.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>