net: dsa: sja1105: add a translation table for port speeds

In order to support the new speed of 2500Mbps, the SJA1110 has achieved
the great performance of changing the encoding in the MAC Configuration
Table for the port speeds of 10, 100, 1000 compared to SJA1105.

Because this is a common driver, we need a layer of indirection in order
to program the hardware with the right values irrespective of switch
generation.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>