pinctrl: Forward declare struct device

pinctrl/devinfo.h is using forward declaration from pinctrl/consumer.h
for configurations with CONFIG_PINCTRL defined, however nothing declares
it in the opposite case. Fix this by adding a forward declaration.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>