gpio: Drop the parent_irq from gpio_irq_chip

We already have an array named "parents" so instead
of letting one point to the other, simply allocate a
dynamic array to hold the parents, just one if desired
and drop the number of members in gpio_irq_chip by
1. Rename gpiochip to gc in the process.

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