openfirmware: Add OF phylib support code

Add support for parsing the device tree for PHY devices on an MDIO bus.
Currently many of the PowerPC ethernet drivers are open coding a solution
for reading data out of the device tree to find the correct PHY device.
This patch implements a set of common routines to:

a) let MDIO bus drivers register phy_devices described in the tree, and
b) let MAC drivers find the correct phy_device via the tree.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index f821dbc..6fe043b 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -19,3 +19,9 @@
 	depends on OF && PPC_OF && SPI
 	help
 	  OpenFirmware SPI accessors
+
+config OF_MDIO
+	def_tristate PHYLIB
+	depends on OF && PHYLIB
+	help
+	  OpenFirmware MDIO bus (Ethernet PHY) accessors