pinctrl: mcp23s08: add open drain configuration for irq output

The mcp23s08 series device can be configured for wired and interrupts
using an external pull-up and open drain output via the IOCON_ODR bit.
And "drive-open-drain" property to enable this.

Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>