dt-bindings: net: Add the Lantiq PEF2256 E1/T1/J1 framer

The Lantiq PEF2256 is a framer and line interface component designed to
fulfill all required interfacing between an analog E1/T1/J1 line and the
digital PCM system highway/H.100 bus.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20231128132534.258459-3-herve.codina@bootlin.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>