pinctrl: armada-37xx: Reuse GPIO fwnode in armada_37xx_irqchip_register()

Since we have fwnode of the first found GPIO controller assigned to the
struct gpio_chip, we may reuse it in the armada_37xx_irqchip_register().

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