net/fsl: xgmac_mdio: Use managed device resources

All of the resources used by this driver has managed interfaces, so
use them. Heed the warning in the comment before platform_get_resource
and use a bare devm_ioremap to allow for non-exclusive access to the
IO memory.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>