net: dsa: b53: mmap: allow passing a chip ID

BCM6318 and BCM63268 SoCs require a special handling for their RGMIIs, so we
should be able to identify them as a special BCM63xx switch.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>