of/net: Move of_get_mac_address() to a common source file.

There are two identical implementations of of_get_mac_address(), one
each in arch/powerpc/kernel/prom_parse.c and
arch/microblaze/kernel/prom_parse.c.  Move this function to a new
common file of_net.{c,h} and adjust all the callers to include the new
header.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
[grant.likely@secretlab.ca: protect header with #ifdef]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index aa675eb..e4b93a0 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -49,6 +49,10 @@
 	help
 	  OpenFirmware I2C accessors
 
+config OF_NET
+	depends on NETDEVICES
+	def_bool y
+
 config OF_SPI
 	def_tristate SPI
 	depends on SPI && !SPARC