ppc: Update README about the new GOT ptr.

r14 is no longer used as non volatile GOT ptr. Instead
the volatile r12 is used so be sure to do GET_GOT in
asm code when you need to access global data.
Signed-off-by: NJoakim Tjernlund <Joakim.Tjernlund@transmode.se>

README

@@ -3922,7 +3922,9 @@ For PowerPC, the following registers have specific use:
 	R30:	GOT pointer
 	R31:	frame pointer
 
-	(U-Boot also uses R14 as internal GOT pointer.)
+	(U-Boot also uses R12 as internal GOT pointer. r12
+	is a volatile register so r12 needs to be reset when
+	going back and forth between asm and C)
 
     ==> U-Boot will use R2 to hold a pointer to the global data