net: ag71xx: Add missing header

ag71xx uses devm_ioremap_nocache. This fixes usage of an implicit function

Fixes: d51b6ce4

 ("net: ethernet: add ag71xx driver")
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>