dt-bindings: arm: Convert QEMU fw-cfg to DT schema

Convert the QEMU fw-cfg binding to DT schema format. As this binding is
also used on Risc-V now, drop any architecture references and move to a
common location. The fw-cfg interface has also gained some DMA support
which is coherent, so add the missing 'dma-coherent'.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Link: https://lore.kernel.org/r/20220310013552.549590-1-robh@kernel.org