net/phy: add trace events for mdio accesses

Make it possible to generate trace events for mdio read and write accesses.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: David S. Miller <davem@davemloft.net>