xen/arm: vgic-v3: Set stride during domain initialization

xen/arm: vgic-v3: Set stride during domain initialization

The stride may not be set if the hardware GIC is using the default
layout. It happens on the Foundation model.

On GICv3, the default stride is 2 * 64K. Therefore it's possible to avoid
checking at every redistributor MMIO access if the stride is not set.

Because domU uses a static stride configuration this only happens for
dom0, so we can move this code in gicv_v3_init. Take the opportunity to move
the stride setting a bit earlier because the loop to set regions will require
the stride.

Also, use 2 * 64K rather than 128K and explain the reason.

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