dt-bindings: arm: Add Versatile Express and Juno YAML schema

This implements the top-level schema for the ARM Versatile
Express and Juno platforms.

Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>