libfdt: Make fdt_string() return a const pointer

Currently, fdt_string() returns a (non-const) char *, despite taking a
const void *fdt.  This is inconsistent with all the other read-only
functions which all return const pointers into the blob.

This patch fixes that.

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

libfdt/fdt_ro.c

@@ -82,7 +82,7 @@ static int nodename_eq(const void *fdt, int offset,
 		return 0;
 }
 
-char *fdt_string(const void *fdt, int stroffset)
+const char *fdt_string(const void *fdt, int stroffset)
 {
 	return (char *)fdt + fdt_off_dt_strings(fdt) + stroffset;
 }

libfdt/libfdt.h

@@ -110,7 +110,7 @@ static inline void *fdt_offset_ptr_w(void *fdt, int offset, int checklen)
 int fdt_move(const void *fdt, void *buf, int bufsize);
 
 /* Read-only functions */
-char *fdt_string(const void *fdt, int stroffset);
+const char *fdt_string(const void *fdt, int stroffset);
 
 int fdt_get_mem_rsv(const void *fdt, int n, uint64_t *address, uint64_t *size);
 int fdt_num_mem_rsv(const void *fdt);