gpio: aspeed: Setup irqchip dynamically

This is in preparation for adding ast2600 support. The ast2600 SoC
requires two instances of the GPIO driver as it has two GPIO
controllers. Each instance needs it's own irqchip.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Link: https://lore.kernel.org/r/20190906062644.13445-1-rashmica.g@gmail.com


Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>