driver/ddr: Restruct driver to allow standalone memory space

U-boot has been initializing DDR for the main memory. The presumption
is the memory stays as a big continuous block, either linear or
interleaved. This change is to support putting some DDR controllers
to separated space without counting into main memory. The standalone
memory controller could use different number of DIMM slots.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/README b/README
index 70dd97d..573666e 100644
--- a/README
+++ b/README
@@ -538,6 +538,12 @@
 		interleaving mode, handled by Dickens for Freescale layerscape
 		SoCs with ARM core.
 
+		CONFIG_SYS_FSL_DDR_MAIN_NUM_CTRLS
+		Number of controllers used as main memory.
+
+		CONFIG_SYS_FSL_OTHER_DDR_NUM_CTRLS
+		Number of controllers used for other than main memory.
+
 - Intel Monahans options:
 		CONFIG_SYS_MONAHANS_RUN_MODE_OSC_RATIO