mtd: of_parts: fix breakage in Kconfig

mtd: of_parts: fix breakage in Kconfig

commit 2e929d001e85126d9267de373d4b76014789661d upstream.

MTD_OF_PARTS and the default setting is not working due to using 'Y'
instead of 'y', introduced in commit
d6137badeff1ef64b4e0092ec249ebdeaeb3ff37. 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>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>