gpio: mmio: Add compatible for the ixp4xx eb MMIO

The IXP4xx expansion bus can have simple memory-mapped GPIO
on it. Using the proper device tree bindings, support probing
this directly from the device tree.

Signed-off-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpio-mmio.c

···
 	{ .compatible = "brcm,bcm6345-gpio" },
 	{ .compatible = "wd,mbl-gpio" },
 	{ .compatible = "ni,169445-nand-gpio" },
+	{ .compatible = "intel,ixp4xx-expansion-bus-mmio-gpio" },
 	{ }
 };
 MODULE_DEVICE_TABLE(of, bgpio_of_match);