net: dsa: b53: prevent FAST_AGE access on BCM5325

BCM5325 doesn't implement FAST_AGE registers so we should avoid reading or
writing them.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20250614080000.1884236-7-noltari@gmail.com


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