mux: mmio: add generic regmap bitfield-based multiplexer

Generic register bitfield-based multiplexer that controls the multiplexer
producer defined under a parent node.
The driver corresponding to parent node provides register read/write
capabilities.

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>