These have now landed upstream. The naming is different and in one case the
function signature has changed. Update the code to match.
This applies the following upstream commits by
Thierry Reding <email@example.com> :
604e61e fdt: Add functions to retrieve strings
8702bd1 fdt: Add a function to get the index of a string
2218387 fdt: Add a function to count strings
Signed-off-by: Simon Glass <firstname.lastname@example.org>