net: mvmdio: new Marvell MDIO driver

This patch adds a separate driver for the MDIO interface of the
Marvell Ethernet controllers. There are two reasons to have a separate
driver rather than including it inside the MAC driver itself:

 *) The MDIO interface is shared by all Ethernet ports, so a driver
    must guarantee non-concurrent accesses to this MDIO interface. The
    most logical way is to have a separate driver that handles this
    single MDIO interface, used by all Ethernet ports.

 *) The MDIO interface is the same between the existing mv643xx_eth
    driver and the new mvneta driver. Even though it is for now only
    used by the mvneta driver, it will in the future be used by the
    mv643xx_eth driver as well.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/marvell/Kconfig b/drivers/net/ethernet/marvell/Kconfig
index 0029934..232ccb3 100644
--- a/drivers/net/ethernet/marvell/Kconfig
+++ b/drivers/net/ethernet/marvell/Kconfig
@@ -31,6 +31,17 @@
 	  Some boards that use the Discovery chipset are the Momenco
 	  Ocelot C and Jaguar ATX and Pegasos II.
 
+config MVMDIO
+	tristate "Marvell MDIO interface support"
+	---help---
+	  This driver supports the MDIO interface found in the network
+	  interface units of the Marvell EBU SoCs (Kirkwood, Orion5x,
+	  Dove, Armada 370 and Armada XP).
+
+	  For now, this driver is only needed for the MVNETA driver
+	  (used on Armada 370 and XP), but it could be used in the
+	  future by the MV643XX_ETH driver.
+
 config PXA168_ETH
 	tristate "Marvell pxa168 ethernet support"
 	depends on CPU_PXA168