arm64: dts: NS2: Add Broadcom SPU driver DT entry

Add Northstar2 device tree entry for Broadcom Secure Processing Unit
(SPU) crypto hardware.

Signed-off-by: Steve Lin <steven.lin1@broadcom.com>
Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>