of/irq: Export of_irq_get()

The function will be used by the I2C core which can be compiled as a
module.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>