pinctrl: st: Include the right header

The ST pinctrl driver wants to provode a gpio_chip but is not
including the header for this, fix the inclusion to use the right
header. <linux/of_gpio.h> has to remain as the driver is calling
of_get_named_gpio().

Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20190820111135.10701-1-linus.walleij@linaro.org