pinctrl: mcp23s08: generalize irq property handling

This moves irq property handling from spi/i2c specific code into
the generic mcp23s08_probe_one. This is possible because the
device properties are named equally.

As a side-effect this drops support for setting the properties via
pdata, which has no mainline users. If boardcode wants to enable
the chip as interrupt controller it can attach the device properties
instead.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>