doc: Document how to apply fdt overlays

We have the capability to apply overlays on the command line but
we didn't have a document explaining how.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Reviewed-by: Ɓukasz Majewski
Acked-by: Simon Glass <sjg@chromium.org>
1 file changed