mfd: intel_quark_i2c_gpio: Convert I²C to use software nodes

The driver can provide a software node group instead of
passing legacy platform data. This will allow to drop
the legacy platform data structures along with unifying
a child device driver to use same interface for all
property providers, i.e. Device Tree, ACPI, and board files.

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