net: dsa: Allow compiling out legacy support

Introduce a configuration option: CONFIG_NET_DSA_LEGACY allowing to compile out
support for the old platform device and Device Tree binding registration.
Support for these configurations is scheduled to be removed in 4.17.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/dsa/Kconfig b/drivers/net/dsa/Kconfig
index 83a9bc8..2b81b97 100644
--- a/drivers/net/dsa/Kconfig
+++ b/drivers/net/dsa/Kconfig
@@ -33,7 +33,7 @@
 
 config NET_DSA_MV88E6060
 	tristate "Marvell 88E6060 ethernet switch chip support"
-	depends on NET_DSA
+	depends on NET_DSA && NET_DSA_LEGACY
 	select NET_DSA_TAG_TRAILER
 	---help---
 	  This enables support for the Marvell 88E6060 ethernet switch