pinctrl: sunxi: introduce IRQ bank conversion function

The Allwinner H6 SoC have its pin controllers with the first IRQ-capable
GPIO bank at IRQ bank 1 and the second bank at IRQ bank 5. Some
refactors in the sunxi pinctrl framework are needed.

This commit introduces a IRQ bank conversion function, which replaces
the "(bank_base + bank)" code in IRQ register access.

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>