gpio: davinci: fix build warning when !CONFIG_OF

This nukes the following warning that is seen when building without
OF support:

drivers/gpio/gpio-davinci.c:437:25: warning: ‘keystone_gpio_get_irq_chip’ defined but not used [-Wunused-function]
 static struct irq_chip *keystone_gpio_get_irq_chip(unsigned int irq)
                         ^~~~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>