net: dsa: b53: mmap: fix device tree support

CPU port should also be enabled in order to get a working switch.

Fixes: a5538a77 ("net: dsa: b53: mmap: Add device tree support")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230316172807.460146-1-noltari@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>