net: dsa: add Broadcom tag RX/TX handler

Add support for the 4-bytes Broadcom tag that built-in switches such as
the Starfighter 2 might insert when receiving packets, or that we need
to insert while targetting specific switch ports. We use a fake local
EtherType value for this 4-bytes switch tag: ETH_P_BRCMTAG to make sure
we can assign DSA-specific network operations within the DSA drivers.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
index f5eede1..a585fd6 100644
--- a/net/dsa/Kconfig
+++ b/net/dsa/Kconfig
@@ -12,6 +12,9 @@
 if NET_DSA
 
 # tagging formats
+config NET_DSA_TAG_BRCM
+	bool
+
 config NET_DSA_TAG_DSA
 	bool