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/Makefile b/drivers/net/Makefile
index 80420f6..dcd5f15 100644
--- a/drivers/net/Makefile
+++ b/drivers/net/Makefile
@@ -95,6 +95,7 @@
 #
 
 obj-$(CONFIG_MII) += mii.o
+obj-$(CONFIG_MDIO) += mdio.o
 obj-$(CONFIG_PHYLIB) += phy/
 
 obj-$(CONFIG_SUNDANCE) += sundance.o