pinctrl: at91: use kernel-doc style for documentation of at91_gpio_chip

Use kernel-doc style for documentation of struct at91_gpio_chip.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220831135636.3176406-2-claudiu.beznea@microchip.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>