ARM: OMAP2+: Prepare to move GPMC to drivers by platform data header

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>