dt-bindings: add mmio-based syscon mux controller DT bindings

This adds device tree binding documentation for mmio-based syscon
multiplexers controlled by a bitfields in a syscon register range.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>