pinctrl: sunxi: Fix multi bank interrupt support in gpio_to_irq

When mapping the interrupts, the gpio_to_irq function did not consider
the bank number of the gpio pin in question, only the offset or the
interrupt number in the bank. As a result, requests for interrupts in
the later banks get mapped to the first bank.

This issue was discovered while enabling mmc on the new sun8i platform.
The tablet I have uses a pin/interrupt from the second bank to do mmc
card detection. Tested on this very device with register inspection and
actual mmc card insertion/removal.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>