libfdt: add stringlist functions to linker script

The fdt_stringlist_count, fdt_stringslist_search, and fdt_stringlist_get
are added to the libfdt linker script as global symbols

Signed-off-by: Reiner Huober <reiner.huober@nokia.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>