mtd: partitions: Add AFS partitions DT bindings

This adds device tree bindings for ARM Firmware Suite
flash partitioning used in NOR flashes on ARM reference
designs.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Richard Weinberger <richard@nod.at>