ARM: OMAP2+: Add missing CONFIG_MTD_NAND_OMAP_BCH to omap2plus_defconfig.

Now that some boards added nand support could be interesting enable the
CONFIG_MTD_NAND_OMAP_BCH feature, otherwise the probe call to nand fails
with following error:

[    1.794527] CONFIG_MTD_NAND_OMAP_BCH is not enabled
[    1.799870] omap2-nand: probe of omap2-nand.0 failed with error -22

Signed-off-by: Enric Balletbo i Serra <eballetbo@iseebcn.com>