gpio/gpio-tps65910: gpio_chip.of_node referenced without CONFIG_OF_GPIO defined

commit 626f9914

 added code to initialize gpio_chip.of_node, but if
CONFIG_OF_GPIO is not defined gps-tps65910 fails to build with an
error complaining gpio_chip has no member of_node. I ran into this
while doing a allyesconfig build on linux-next.

Signed-off-by: Gerard Snitselaar <dev@snitselaar.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>