net: phy: mscc-miim: Add MDIO driver

Add a driver for the Microsemi MII Management controller (MIIM) found on
Microsemi SoCs.
On Ocelot, there are two controllers, one is connected to the internal
PHYs, the other one can communicate with external PHYs.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/Kconfig

@@ -123,6 +123,13 @@ config MDIO_MOXART
 	  This driver supports the MDIO interface found in the network
 	  interface units of the MOXA ART SoC
 
+config MDIO_MSCC_MIIM
+	tristate "Microsemi MIIM interface support"
+	depends on HAS_IOMEM
+	help
+	  This driver supports the MIIM (MDIO) interface found in the network
+	  switches of the Microsemi SoCs
+
 config MDIO_OCTEON
 	tristate "Octeon and some ThunderX SOCs MDIO buses"
 	depends on 64BIT