doc: sphinx: Add Pygment lexer for DTS

Add pygment lexer for DTS code-block.

Fixes #6029

Origin: skiboot, https://github.com/qemu/skiboot
Status:  91350c5a926795d8917a4eff699941361f780476
Description: Syntax highlighting of Device Tree Source (dts)
Dependencies: None
URL: https://github.com/qemu/skiboot/blob/master/doc/DtsLexer.py
commit: 91350c5a926795d8917a4eff699941361f780476
Maintained-by: External
License: BSL-1.0
License Link:
    https://github.com/qemu/skiboot/blob/master/doc/DtsLexer.py



Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>