ppc: Fix ppc4xx CONFIG_SYS_GENERIC_BOARD

I realized that with v2014.10 u-boot is not starting up on
ppc4xx boards with CONFIG_SYS_GENERIC_BOARD set.

I bisected it down to this:
d54d7eb9 support blackfin board initialization in generic board_f

With
d29437aa ppc: Make ppc4xx ready for CONFIG_SYS_GENERIC_BOARD
I set gd to a defined value for ppc4xx in cpu_init_f().
I did not realize that the gd struct has also to be memset()
to zero at this point.

But at least commit d54d7eb9 assumes it is:
@@ -516,11 +528,13 @@ static int reserve_malloc(void)
 /* (permanently) allocate a Board Info struct */
 static int reserve_board(void)
 {
-	gd->start_addr_sp -= sizeof(bd_t);
-	gd->bd = (bd_t *)map_sysmem(gd->start_addr_sp, sizeof(bd_t));
-	memset(gd->bd, '\0', sizeof(bd_t));
-	debug("Reserving %zu Bytes for Board Info at: %08lx\n",
-			sizeof(bd_t), gd->start_addr_sp);
+	if (!gd->bd) {
+		gd->start_addr_sp -= sizeof(bd_t);
+		gd->bd = (bd_t *)map_sysmem(gd->start_addr_sp, sizeof(bd_t));
+		memset(gd->bd, '\0', sizeof(bd_t));
+		debug("Reserving %zu Bytes for Board Info at: %08lx\n",
+		      sizeof(bd_t), gd->start_addr_sp);
+	}
 	return 0;
 }
 #endif

This might also be an issue on other architectures, so maintainers should
check.

Signed-off-by: Dirk Eibach <dirk.eibach@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>