pinctrl: sunxi: refactor irq related register function to have desc

As the new H6 SoC has holes in the IRQ registers, refactor the IRQ
related register function for getting the full pinctrl desc structure.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>