doc: Update devicedocs including how to add tweaks

This file is about 10 years old and the updates have not covered
everything that has changed, particularly in the last few years. Update
the information and add mention of the u-boot.dtsi files.

Signed-off-by: Simon Glass <sjg@chromium.org>

Fix typos.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
1 file changed