pca954x: harmonize pca9547 and pca954x and add pca9540 support

The two implementations for the pca9548 switch and the pca9547 mux
seemed close enough so we can put them together and with a bit more
abstraction add pca9540 support.

While here apply a bit of consistency in variable and driver naming and
use device_has_property instead of the FDT-only OF_ variant.

This disconnects pca9547 from the build but does not yet delete it.

MFC after: 2 weeks
Reviewed by: mmel (earlier version), avg
Differential Revision: https://reviews.freebsd.org/D35701