dt-bindings: marvell: Rewrite MV88E6xxx in schema

This is an attempt to rewrite the Marvell MV88E6xxx switch bindings
in YAML schema.

The current text binding says:
  WARNING: This binding is currently unstable. Do not program it into a
  FLASH never to be changed again. Once this binding is stable, this
  warning will be removed.

Well that never happened before we switched to YAML markup,
we can't have it like this, what about fixing the mess?

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>