gpio: altera: Allocate irq_chip dynamically

Keeping the irq_chip definition static shares it with multiple instances
of the altera gpiochip in the system. This is bad and now we get this
warning from gpiolib core:

"detected irqchip that is shared with multiple gpiochips: please fix the
driver."

Hence, move the irq_chip definition from being driver static into the
struct altera_gpio_chips. So a unique irq_chip is used for each gpiochip
instance.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>