gpio: pcf857x: Add IRQF_SHARED when request irq

It's quite possible that multiple pcf857x can be hooked up
to the same interrupt line with the processor. So add IRQF_SHARED
in request irq..

Signed-off-by: George Cherian <george.cherian@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>