This driver is implementing a GPIO driver so include
<linux/gpio/driver.h> and not the legacy API <linux/gpio.h>.
When testing it turns out it also relies on implicit
inclusion of <linux/io.h> (readw etc) so make sure to
include that as well.

Cc: Chris Brandt <chris.brandt@renesas.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>