xen/arm: vgic: Override the group in lr everytime

xen/arm: vgic: Override the group in lr everytime

At the moment, write_lr is assuming the caller will set correctly the
group. However the group should always be 0 when the guest is using
vGICv2 and 1 for vGICv3. As the caller should not care about the group,
override it directly.

With that change, write_lr is now behaving like update_lr for the group.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>