PPC405EX CHIP_21 erratum

APM errata CHIP_21 for the 405EX/EXr (from the rev 1.09 document dated
4/27/11) states that rev D processors may wake up with the wrong feature
set.  This patch implements the APM-proposed workaround.

To enable this patch for your board, add the appropriate define for your
CPU to your board header file.  See kilauea.h for more information.  The
following variants are supported:

#define CONFIG_SYS_4xx_CHIP_21_405EX_NO_SECURITY
#define CONFIG_SYS_4xx_CHIP_21_405EX_SECURITY
#define CONFIG_SYS_4xx_CHIP_21_405EXr_NO_SECURITY
#define CONFIG_SYS_4xx_CHIP_21_405EXr_SECURITY

Please note that if you select the wrong define, your board will not
boot, and JTAG will be required to recover.

Tested on custom boards using:

CONFIG_SYS_4xx_CHIP_21_405EX_NO_SECURITY  <sfalco@harris.com>
CONFIG_SYS_4xx_CHIP_21_405EX_SECURITY     <eibach@gdsys.de>

Signed-off-by: Steve Falco <sfalco@harris.com>
Acked-by: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: Stefan Roese <sr@denx.de>