fbdev/sh_mobile: Drop unused include

The driver includes <linux/gpio.h> but doesn't use any symbols
from this file.

Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: linux-renesas-soc@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be> # on r8a77440/armadillo, where this is the last piece of non-DT support.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201030002850.6495-1-linus.walleij@linaro.org