net: dsa: sja1105: allow the frame buffer size to be customized

The shared frame buffer of the SJA1110 is larger than that of SJA1105,
which is natural due to the fact that there are more ports.

Introduce yet another property in struct sja1105_info which encodes the
maximum number of 128 byte blocks that can be used for frame buffers.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>