gpio: regmap: Switch to use fwnode instead of of_node

GPIO library now accepts fwnode as a firmware node, so
switch the driver to use it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>