pinctrl: bcm2835: Remove unneeded irq_group field

The irq_group field stores a 1:1 mapping. Use the loop variable to
derive the values instead of storing them in an extra array.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>