dt-bindings: add register based devices' mux controller DT bindings

This adds device tree binding documentation for generic register based
multiplexer controlled by a bitfields in a parent device's register range.

since MMIO mux is a special case of generic register based mux, the
MMIO mux bindings have been subsumed in these bindings.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>