mtd: nand: omap: move omap_elm.h from arch/arm/include/asm to drivers/mtd/nand

omap_elm.h is a generic header used by OMAP ELM driver for all TI platfoms.
Hence this file should be present in generic folder instead of architecture
specific include folder.
Build tested using: ./MAKEALL -s am33xx -s omap3 -s omap4 -s omap5

Signed-off-by: Pekon Gupta <pekon@ti.com>