mtd: nand: omap: make am33xx/elm.c as common driver for all OMAPx and AMxxxx platforms

ELM hardware engine which is used for ECC error detection, is present on all
latest OMAP SoC (like OMAP4xxx, OMAP5xxx, DRA7xxx, AM33xx, AM43xx). Thus ELM
driver should be moved to common drivers/mtd/nand/ folder so that all SoC
having on-chip ELM hardware engine can re-use it.
This patch has following changes:
- mv arch/arm/include/asm/arch-am33xx/elm.h arch/arm/include/asm/omap_elm.h
- mv arch/arm/cpu/armv7/am33xx/elm.c drivers/mtd/nand/omap_elm.c
- update Makefiles
- update #include <asm/elm.h>
- add CONFIG_NAND_OMAP_ELM to compile driver/mtd/nand/omap_elm.c
	and include in all board configs using AM33xx SoC platform.

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