Merge branch 'b53-docs'

Benedikt Spranger says:

====================
Document the configuration of b53

this is the third round to document the configuration of a b53 supported
switch.

v3..v2:
- fix a typo
- improve b53 configuration in DSA_TAG_PROTO_NONE showcase.
- grade up from RFC to patch for mainline inclusion.

v1..v2:
- split out generic parts of the configuration.
- target comments by Andrew Lunn and Florian Fainelli.
- make changes visible to build system
====================

Signed-off-by: David S. Miller <davem@davemloft.net>