devicetree: update documentation for fw_cfg ARM bindings

Remove fw_cfg hardware interface details from
Documentation/devicetree/bindings/arm/fw-cfg.txt,
and replace them with a pointer to the authoritative
documentation in the QEMU source tree.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Cc: Laszlo Ersek <lersek@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>