Xen: ARM: Zero reserved fields of xatp before making hypervisor call

Ensure all reserved fields of xatp are zero before making
hypervisor call to XEN in xen_map_device_mmio().
xenmem_add_to_physmap_one() in XEN fails the mapping request if
extra.res reserved field in xatp is not zero for XENMAPSPACE_dev_mmio
request.

Signed-off-by: Jiandi An <anjiandi@codeaurora.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>