gpio: gpio-aspeed-sgpio: Move irq_chip to aspeed-sgpio struct

The current design initializes irq->chip from a global irqchip struct,
which causes multiple sgpio devices use the same irq_chip.
The patch moves irq_chip to aspeed_sgpio struct for initializing
irq_chip from their private gpio struct.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>