dt-bindings: mtd: describe the simple BCM963XX NOR flash layout

Add binding documentation for the standard CFE based BCM963XX flash
layout, found in most devices using a BCM63XX SoC with NOR flash.

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>