bridge: cfm: Add BRIDGE_CFM to Kconfig.

This makes it possible to include or exclude the CFM
protocol according to 802.1Q section 12.14.

Signed-off-by: Henrik Bjoernlund <henrik.bjoernlund@microchip.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>