gpio/omap: fix build error when OF_GPIO is not defined.

The OMAP GPIO driver check if the chip has an associated
Device Tree node using the struct gpio_chip of_node member.

But this is only build if CONFIG_OF_GPIO is defined which
leads to the following error when using omap1_defconfig:

linux/drivers/gpio/gpio-omap.c: In function 'omap_gpio_chip_init':
linux/drivers/gpio/gpio-omap.c:1080:17: error: 'struct gpio_chip' has no member named 'of_node'
linux/drivers/gpio/gpio-omap.c: In function 'omap_gpio_irq_map':
linux/drivers/gpio/gpio-omap.c:1116:16: error: 'struct gpio_chip' has no member named 'of_node'

Reported-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>