Fix some errors in comments

A comment in tests/stringlist.c refers to fdt_get_string(), which is not a
function that exists.  From the content, it's supposed to be referring to
fdt_getprop_string().

A comment in libfdt.h has an extraneous space in a function name.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/libfdt.h

@@ -1449,7 +1449,7 @@ int fdt_setprop(void *fdt, int nodeoffset, const char *name,
 		const void *val, int len);
 
 /**
- * fdt_setprop _placeholder - allocate space for a property
+ * fdt_setprop_placeholder - allocate space for a property
  * @fdt: pointer to the device tree blob
  * @nodeoffset: offset of the node whose property to change
  * @name: name of the property to change