mfd: intel-lpss: Switch to use the software nodes

Software node was always created for the device if it was
supplied with additional device properties, so those nodes
might as well be constant.

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>