gpio/omap: auto-setup a GPIO when used as an IRQ

The OMAP GPIO controller HW requires a pin to be configured in GPIO
input mode in order to operate as an interrupt input. Since drivers
should not be aware of whether an interrupt pin is also a GPIO or not,
the HW should be fully configured/enabled as an IRQ if a driver solely
uses IRQ APIs such as request_irq(), and never calls any GPIO-related
APIs. As such, add the missing HW setup to the OMAP GPIO controller's
irq_chip driver.

Since this bypasses the GPIO subsystem we have to ensure that another
driver won't be able to request the same GPIO pin that is used as an
IRQ and set its direction as output. Requesting the GPIO and setting
its direction as input is allowed though.

This fixes smsc911x ethernet support for tobi and igep OMAP3 boards
and OMAP4 SDP SPI based ethernet that use a GPIO as an interrupt line.

Cc: stable@vger.kernel.org
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: George Cherian <george.cherian@ti.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Tested-by: Lars Poeschel <poeschel@lemonage.de>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>