doc: Add man page for dumpimage

This tool seems a bit underloved. Unfortunately, it seems to be missing
support for FIT images. Alas...

Add a man page documenting it. The example is taken from commit a804b5ce
("Add dumpimage, a tool to extract data from U-Boot images").

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>