pinctrl: single: Clear pin interrupts enabled by bootloader

Since we set up device wake-up interrupts as pinctrl-single
interrupts, we now must use the standard request_irq and
related functions to manage them.

If the pin interrupts are enabled for some pins at boot,
the wake-up events can show up as constantly pending
at least on omaps and will hang the system unless the related
device driver clears the event at the device.

To fix this, let's clear the interrupt flags during init,
and print out a warning so the board maintainers can update
their drivers to do proper request_irq for the driver specific
wake-up events.

Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>