net: at91_ether: add pinctrl support

If no pinctrl available just report a warning as some architecture may not
need to do anything.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Tested-by: Joachim Eastwood <manabian@gmail.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>