ARM: OMAP: Move gpio.h to include/linux/platform_data

This way we can remove includes of plat/gpio.h which won't work
with the single zImage support.

Note that we also remove the cpu_class_is_omap2() check
in gpio-omap.c as the drivers should not call it as we need to
make it local to arch/arm/mach-omap2 for single zImage support.

While at it, arrange the related includes in the standard way.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: linux-mtd@lists.infradead.org
Cc: alsa-devel@alsa-project.org
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>