gpiolib: of: constify few local device_node variables

gpiolib does not modify struct device_node, so few local pointers can
point to a const data.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>