iicbus/mux/pca954x: add support for PCA9546 I2C Switch

Add support for the 4 channel I2C switch from NXP by adding a new
description struct and the list entries. Compared to x=[2345] which
require code to support the INT, for this one no further changes are
needed.

Tested on: WHLE-LS1088A using a SPF+
MFC after: 1 week
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44009