mtd: of_parts: fix breakage in Kconfig

mtd: of_parts: fix breakage in Kconfig

MTD_OF_PARTS and the default setting is not working due to using 'Y'
instead of 'y', introduced in commit
40d9bc610b67b0df4421b20bbc1627c86adbcdb8. This made our board, and
possibly other boards using DTS defined partitions and not having
CONFIG_MTD_OF_PARTS=y defined in the defconfig, fail to mount root.

Signed-off-by: Frank Svendsboe <frank.svendsboe@gmail.com>
Cc: stable@kernel.org [3.2+]
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>