mpc8xxx: Enable ECC on/off control in hwconfig

Add fsl_ddr:ecc=on in hwconfig. If ECC is enabled in board configuration file,
ECC can be turned on/off by this switch. If this switch is omitted, it is ON by
default.

Updated hwconfig calls to use local buffer.

Syntax is
hwconfig=fsl_ddr:ecc=on

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
diff --git a/doc/README.fsl-ddr b/doc/README.fsl-ddr
index 1657ef6..9e3c539 100644
--- a/doc/README.fsl-ddr
+++ b/doc/README.fsl-ddr
@@ -78,6 +78,13 @@
 Syntax is:
 hwconfig=fsl_ddr:addr_hash=true
 
+Memory controller ECC on/off
+============================
+If ECC is enabled in board configuratoin file, i.e. #define CONFIG_DDR_ECC,
+ECC can be turned on/off by hwconfig.
+
+Syntax is
+hwconfig=fsl_ddr:ecc=off
 
 Memory testing options for mpc85xx
 ==================================