Documentation: dontdiff: Update with additional entries

Add a bunch of entries reflective of programs that the kernel build:
sortextable, dtc. And while at it, expand the lex*.c entries to cover
e.g: dtc-lexer.c. Finally, exclude devicetable-offsets.h

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>