pinctrl: Add support for the Lantic PEF2256 pinmux

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.

This kind of component can be found in old telecommunication system.
It was used to digital transmission of many simultaneous telephone calls
by time-division multiplexing. Also using HDLC protocol, WAN networks
can be reached through the framer.

This pinmux support handles the pin muxing part (pins RP(A..D) and pins
XP(A..D)) of the PEF2256.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20231128132534.258459-5-herve.codina@bootlin.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>