emev2: GPIOLIB: Enable support for OF

EMEV2 is now a DT platform, however the GPIO driver cannot be used
from a DT file since it does not fill out the of_node field in its
gpio_chip structure.

Signed-off-by: Ian Molton <ian.molton@codethink.co.uk>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>