memory: omap-gpmc: Move device tree binding to correct location

omap-gpmc.c is a memory controller so move the binding to the
right place.
Signed-off-by: NRoger Quadros <rogerq@ti.com>
Acked-by: NRob Herring <robh@kernel.org>
Acked-by: NTony Lindgren <tony@atomide.com>