ARM: dts: NSP: Switch to port 8 for CPU port

Now that we have added support for pre-pended Broadcom tags with commit
11606039 ("net: dsa: b53: Support prepended Broadcom tags") we can
switch all the Northstar Plus reference boards to use port 8 for the CPU
port. This allows us to prepare room for supporting the Flow Accelerator
2 NAPT offload, and frees up port 5 to be made fully configurable for
the modes that it supports: internal, SGMII, RGMII etc.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>