pinctrl: at91: Make use of for_each_requested_gpio()

Make use of for_each_requested_gpio() instead of home grown analogue.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200615150545.87964-6-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>