arm64: dts: Add SMMU DT node for NS2

The SMMU-500 driver is already available in Linux kernel. Let's
enable it for NS2 in DT.

This patch keeps mmu-masters attribute empty so that driver patches
can later extend this attribute when adding device DT nodes.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>