devicetree: document ARM bindings for QEMU's Firmware Config interface

Peter Maydell suggested that we describe new devices / DTB nodes in the
kernel Documentation tree that we expose to arm "virt" guests in QEMU.

Although the kernel is not required to access the fw_cfg interface,
"Documentation/devicetree/bindings/arm" is probably the best central spot
to keep the fw_cfg description in.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>