pinctrl: npcm7xx: Switch to use for_each_gpiochip_node() helper

Switch the code to use for_each_gpiochip_node() helper.

While at it, in order to avoid additional churn in the future,
do the following:
- use a temporary variable for struct device pointer to shorten a few lines
- get rid of a temporary variable for vIRQ number, assign it directly
- switch to fwnode APIs where it makes sense

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>