xen/arm: gic-v3: Fix the distributor region to 64kB

xen/arm: gic-v3: Fix the distributor region to 64kB

On GICv3, the default size of the distributor region is 64kB (see 5.3
in PRD03-GENC-010745 24.0). This region can be extended to provide an
implementation defined set of pages containing additional aliases for MSI.
Although, the GICv3 driver only access to register within the default
distributor region.

Furthermore, our vGIC driver implementation doesn't support the extended
distributor. Therefore there is no reason to expose it to DOM0.

Finally drop the field dbase_size which is not useful anymore.

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