firmware: Add coreboot device tree binding documentation

This patch adds documentation describing a device tree binding for the
coreboot firmware. It is meant to be dynamically added during boot and
contains address definitions for the coreboot table (a list of
variable-sized descriptors providing information about various compile-
and run-time generated firmware parameters) and the CBMEM area (the
structure containing most run-time resident memory regions set up by
coreboot).

These definitions allow kernel drivers to easily access data contained
in and pointed to by these regions (such as coreboot's in-memory log).
(An example implementation can be seen in the following patch)

Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>