pinctrl: mcp23s08: Allocate irq_chip dynamic

Keeping the irq_chip definition static shares it with multiple instances
of the mcp23s08 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 mcp23s08. So a unique irq_chip is used for each gpiochip
instance.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>