dt-bindings: arm: Convert BCM2835 board/soc bindings to json-schema

Convert the BCM2835/6/7 SoC bindings to DT schema format using json-schema.
All the other Broadcom boards are maintained by Florian Fainelli.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Herring <robh@kernel.org>