libfdt: Bring in upstream stringlist functions

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 <treding@nvidia.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 <sjg@chromium.org>
diff --git a/drivers/pinctrl/meson/pinctrl-meson.c b/drivers/pinctrl/meson/pinctrl-meson.c
index d21a3dd..46470ea 100644
--- a/drivers/pinctrl/meson/pinctrl-meson.c
+++ b/drivers/pinctrl/meson/pinctrl-meson.c
@@ -122,7 +122,7 @@
 	int index, len = 0;
 	const fdt32_t *reg;
 
-	index = fdt_find_string(gd->fdt_blob, offset, "reg-names", name);
+	index = fdt_stringlist_search(gd->fdt_blob, offset, "reg-names", name);
 	if (index < 0)
 		return FDT_ADDR_T_NONE;