dt: bindings: i2c-mux-pca954x: Add documentation for interrupt controller

Various muxes can aggregate multiple irq lines and provide a control
register to determine the active line. Add bindings for interrupt
controller support.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Peter Rosin <peda@axentia.se>