arm64: dts: Update Broadcom NS2 to generic IOMMU binding

With the "mmu-masters" property now deprecated and optional, the
generic binding offers a more efficient way to specify no masters.

CC: Ray Jui <rjui@broadcom.com>
CC: Scott Branden <sbranden@broadcom.com>
CC: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>