arm/xen: Use xen_xlate_map_ballooned_pages to setup grant table

Use xen_xlate_map_ballooned_pages to setup grant table. Then it doesn't
rely on DT or ACPI to pass the start address and size of grant table.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Tested-by: Julien Grall <julien.grall@arm.com>