net: dsa: add support for bridge flags

The Linux bridge implementation allows various properties of the bridge
to be controlled, such as flooding unknown unicast and multicast frames.
This patch adds the necessary DSA infrastructure to allow the Linux
bridge support to control these properties for DSA switches.

Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
[florian: Add missing dp and ds variables declaration to fix build]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>