dt/bindings: Add bindings for FWU Metadata mtd storage

Add bindings needed for accessing the FWU metadata regions.
These include the compatible string which point to the access
method, the actual device which stores the FWU metadata and
the offsets for both metadata regions.

The current patch adds basic bindings needed for accessing the
metadata structure on non-GPT mtd regions.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>