net: dsa: sja1105: implement a common frame memory partitioning function

There are 2 different features that require some reserved frame memory
space: VLAN retagging and virtual links. Create a central function that
modifies the static config and ensures frame memory is never
overcommitted.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>