xen/arm: if direct-map domain use native addresses for GICv2

xen/arm: if direct-map domain use native addresses for GICv2

Today we use native addresses to map the GICv2 for Dom0 and fixed
addresses for DomUs.

This patch changes the behavior so that native addresses are used for
all domains that are direct-mapped.

NEW VGIC has different naming schemes, like referring distributor base
address as vgic_dist_base, other than the dbase. So this patch also introduces
vgic_dist_base/vgic_cpu_base accessor to access correct distributor base
address/cpu interface base address on varied scenarios,

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>