net: dsa: sja1105: Add a global sja1105_tagger_data structure

This will be used to keep state for RX timestamping. It is global
because the switch serializes timestampable and meta frames when
trapping them towards the CPU port (lower port indices have higher
priority) and therefore having one state machine per port would create
unnecessary complications.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>