driver/ddr: Add 256 byte interleaving support

Freescale LayerScape SoCs support controller interleaving on 256 byte size.
This interleaving is mandoratory.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/README b/README
index 355e898..f51f17e 100644
--- a/README
+++ b/README
@@ -497,6 +497,11 @@
 		same as CONFIG_SYS_DDR_SDRAM_BASE for  all Power SoCs. But
 		it could be different for ARM SoCs.
 
+		CONFIG_SYS_FSL_DDR_INTLV_256B
+		DDR controller interleaving on 256-byte. This is a special
+		interleaving mode, handled by Dickens for Freescale layerscape
+		SoCs with ARM core.
+
 - Intel Monahans options:
 		CONFIG_SYS_MONAHANS_RUN_MODE_OSC_RATIO