fdt: Export fdtdec_lookup() and fix the name

The name of this function is not consistent, so fix it, and export
the function for external use.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 32f03cc..96f3e7b 100644
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -128,7 +128,7 @@
 	return 1;
 }
 
-enum fdt_compat_id fd_dec_lookup(const void *blob, int node)
+enum fdt_compat_id fdtdec_lookup(const void *blob, int node)
 {
 	enum fdt_compat_id id;