gpio: pl061: Move irq_chip definition inside struct pl061

Keeping the irq_chip definition static will make it shared with multiple
giochips in the system. This practice is considered to be bad and now we
will get the below warning from gpiolib core:

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

Hence, move the irq_chip definition from static to `struct pl061` for
using a unique irq_chip for each gpiochip.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>