net: dsa: b53: Support setting learning on port

Add support for being able to set the learning attribute on port, and
make sure that the standalone ports start up with learning disabled.

We can remove the code in bcm_sf2 that configured the ports learning
attribute because we want the standalone ports to have learning disabled
by default and port 7 cannot be bridged, so its learning attribute will
not change past its initial configuration.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>