We still need to support platform data for omap3 until it's booting
in device tree only mode. So let's add platform_data/omap-gpmc.h for
that, and a minimal linux/omap-gpmc.h for the save and restore used
by the PM code.

Let's also keep a minimal mach-omap2/gpmc.h still around to avoid
churn on the board-*.c files. Once omap3 boots in device tree only
mode, we can drop mach-omap2/gpmc.h and we can make the data
structures in platform_data/omap-gpmc.h private to the GPMC driver.

Note that we can now also remove gpmc-nand.h and gpmc-onenand.h.

Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>