ANDROID: GKI: Export memblock_free to drivers

On architectures that support the preservation of memblock metadata
after __init, allow drivers to call memblock_free() to free a
reservation made by early arch code. This is a hack to support the
freeing of bootsplash reservations passed to Linux by the bootloader.

(This should be reworked in future versions of Android; do not
cherry-pick this patch forward.)

Bug: 139653858
Change-Id: I32c0ee70c33c94deff70aa548896caa9978396fb
Signed-off-by: Alistair Delva <adelva@google.com>