asm/omap_gpmc.h: consolidate common defines

arch/arm/include/asm/arch-am33xx/omap_gpmc.h and
arch/arm/include/asm/arch-omap3/omap_gpmc.h are almost the same, consolidate
the common parts into a new header.

Introduce a new asm/omap_gpmc.h which defines the command part and pulls in
the architecture specific one.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Tom Rini <trini@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>