dt-bindings: mtd: describe BCM963XX ImageTag format and usage

Describe how to use the BCM963XX ImageTag format in a mixed flash layout
environment.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>