gpio: aspeed: Always register the irqchip

The driver was implemented in a way that made the irqchip optional, if a
irq was not present in the device tree. However, all of the device trees
have always had an irq, so the optional-ness has never been used.

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