not sure about the reincarnation stuff, birthmarks are sometimes shared by the same members of a family in the exact same place in the body and are the same pattern/symbol.

for me it's sure a mark of something appearing before birth and visible after your skin is almost mature enough to show it (babies don't have freckles until a certain age, the maturation process of our skin is not 100% completed in the womb) but not related to death or reincarnation.