mtd: nand: omap: enable BCH ECC scheme using ELM for generic platform

BCH8_ECC scheme implemented in omap_gpmc.c driver has following favours
+-----------------------------------+-----------------+-----------------+
|ECC Scheme                         | ECC Calculation | Error Detection |
+-----------------------------------+-----------------+-----------------+
|OMAP_ECC_BCH8_CODE_HW              |GPMC             |ELM H/W engine   |
|OMAP_ECC_BCH8_CODE_HW_DETECTION_SW |GPMC             |S/W BCH library  |
+-----------------------------------+-----------------+-----------------+

Current implementation limits the BCH8_CODE_HW only for AM33xx device family.
(using CONFIG_AM33XX). However, other SoC families (like TI81xx) also have
ELM hardware module, and can support ECC error detection using ELM.

This patch
- removes CONFIG_AM33xx
	Thus this driver can be reused by all devices having ELM h/w engine.
- adds omap_select_ecc_scheme()
	A common function to handle ecc-scheme related configurations. This
	can be used both during device-probe and via user-space u-boot commads
	to change ecc-scheme. During device probe ecc-scheme is selected based
	on CONFIG_NAND_OMAP_ELM or CONFIG_NAND_OMAP_BCH8
- enables CONFIG_BCH
	S/W library (lib/bch.c) required by OMAP_ECC_BCHx_CODE_HW_DETECTION_SW
  	is enabled by CONFIG_BCH.
- enables CONFIG_SYS_NAND_ONFI_DETECTION
	for auto-detection of ONFI compliant NAND devices
- updates following README doc
	doc/README.nand
	board/ti/am335x/README
	doc/README.omap3

Signed-off-by: Pekon Gupta <pekon@ti.com>
[scottwood@freescale.com: fixed unused variable warning]
Signed-off-by: Scott Wood <scottwood@freescale.com>