mfd: core: Add support for software nodes

The old device property API is going to be removed and
replaced with the newer software node API. This prepares MFD
subsystem for the transition.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>