pinctrl: fix build when CONFIG_OF && !CONFIG_PINCTRL

pinctrl/devicetree.c won't compile when !CONFIG_PINCTRL, since the
pinctrl headers don't declare some types when !PINCTRL. Make sure
pinctrl/Makefile only attempts to compile devicetree.c when OF &&
PINCTRL.

Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>