ANDROID: GKI: Revert "mm: unexport free_reserved_area"

This reverts commit afa00112.

Vendors would still like to use the free_reserved_area() API.

Bug: 146846822
[saravanak Changed EXPORT_SYMBOL() to EXPORT_SYMBOL_GPL()]
Signed-off-by: Saravana Kannan <saravanak@google.com>
Change-Id: Ic926e79270203cc0614b913d0bfd88aef093cdeb