net: dsa: sja1105: make the AVB table dynamically reconfigurable

The AVB table contains the CAS_MASTER field (to be added in the next
patch) which decides the direction of the PTP_CLK pin.

Reconfiguring this field dynamically is highly preferable to having to
reset the switch and upload a new static configuration, so we add
support for exactly that.

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