mtd: nand: omap: add CONFIG_NAND_OMAP_ECCSCHEME for selection of ecc-scheme

This patch adds new CONFIG_NAND_OMAP_ECCSCHEME, replacing other distributed
CONFIG_xx used for selecting NAND ecc-schemes.
This patch aims at solving following issues.

1) Currently ecc-scheme is tied to SoC platform, which prevents user to select
   other ecc-schemes also supported in hardware. like;
 - most of OMAP3 SoC platforms use only 1-bit Hamming ecc-scheme, inspite
   the fact that they can use higher ecc-schemes like 8-bit ecc-schemes with
   software based error detection (OMAP_ECC_BCH4_CODE_HW_DETECTION_SW).
 - most of AM33xx SoC plaforms use 8-bit BCH ecc-scheme for now, but hardware
   supports BCH16 ecc-scheme also.

2) Different platforms use different CONFIG_xx to select ecc-schemes, which
   adds confusion for user while migrating platforms.
 - *CONFIG_NAND_OMAP_ELM* which enables ELM hardware engine, selects only
    8-bit BCH ecc-scheme with h/w based error-correction (OMAP_ECC_BCH8_CODE_HW)
    whereas ELM hardware engine supports other ecc-schemes also like; BCH4,
    and BCH16 (in future).
 - *CONFIG_NAND_OMAP_BCH8* selects 8-bit BCH ecc-scheme with s/w based error
    correction (OMAP_ECC_BCH8_CODE_HW_DETECTION_SW).
 - *CONFIG_SPL_NAND_SOFTECC* selects 1-bit Hamming ecc-scheme using s/w library

Thus adding new *CONFIG_NAND_OMAP_ECCSCHEME* de-couples ecc-scheme dependency
on SoC platform and NAND driver. And user can select ecc-scheme independently
foreach board.
However, selection some hardware based ecc-schemes (OMAP_ECC_BCHx_CODE_HW) still
depends on presence of ELM hardware engine on SoC. (Refer doc/README.nand)

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