powerpc: fix wrong comment at GOT definitions

r12 is used for accessing the GOT not r14. Fix this in the
comment.
Signed-off-by: NHeiko Schocher <hs@denx.de>

include/ppc_asm.tmpl

@@ -35,7 +35,7 @@
  *
  * Stolen from prepboot/bootldr.h, (C) 1998 Gabriel Paubert, paubert@iram.es
  *
- * Uses r14 to access the GOT
+ * Uses r12 to access the GOT
  */
 
 #define START_GOT			\