gpio: pcf857x: Fix interrupts on multiple instances

When multiple instances of pcf857x chips are present, a fix up
message [1] is printed during the probe of the 2nd and later
instances.

The issue is that the driver is using the same irq_chip data
structure between multiple instances.

Fix this by allocating the irq_chip data structure per instance.

[1] fix up message addressed by this patch
[    1.212100] gpio gpiochip9: (pcf8575): detected irqchip that is shared with multiple gpiochips: please fix the driver.

Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>