gpio: MT7621: use a per instance irq_chip structure

This fixes the kernel complains:
gpio gpiochip1: (1e000600.gpio-bank1): detected irqchip that is shared
	    with multiple gpiochips: please fix the driver.
gpio gpiochip2: (1e000600.gpio-bank2): detected irqchip that is shared
	    with multiple gpiochips: please fix the driver.

Fixes: 4ba9c3af ("gpio: mt7621: Add a driver for MT7621")
Cc: stable@vger.kernel.org
Signed-off-by: René van Dorst <opensource@vdorst.com>
Cc: linux-gpio@vger.kernel.org
Cc: linux-mediatek@lists.infradead.org
Tested-by: Greg Ungerer <gerg@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>