mdio: Add generic MDIO (clause 45) support functions

These roughly mirror many of the MII library functions and are based
on code from the sfc driver.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index bc7eef1..2f81db5 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -2443,6 +2443,9 @@
 
 if NETDEV_10000
 
+config MDIO
+	tristate
+
 config CHELSIO_T1
         tristate "Chelsio 10Gb Ethernet support"
         depends on PCI