armv8/ls2085a: Update SoC README for DDR layout

Update SoC README to provide details of
 - Memory regions
 - Memory used by MC and Debug server

Signed-off-by: Bhupesh Sharma <bhupesh.sharma at freescale.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar at freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>

arch/arm/cpu/armv8/fsl-lsch3/README

@@ -9,6 +9,31 @@ Freescale LayerScape with Chassis Generation 3
 This architecture supports Freescale ARMv8 SoCs with Chassis generation 3,
 for example LS2085A.
 
+DDR Layout
+============
+Entire DDR region splits into two regions.
+ - Region 1 is at address 0x8000_0000 to 0xffff_ffff.
+ - Region 2 is at 0x80_8000_0000 to the top of total memory,
+   for example 16GB, 0x83_ffff_ffff.
+
+All DDR memory is marked as cache-enabled.
+
+When MC and Debug server is enabled, they carve 512MB away from the high
+end of DDR. For example, if the total DDR is 16GB, it shrinks to 15.5GB
+with MC and Debug server enabled. Linux only sees 15.5GB.
+
+The reserved 512MB layout looks like
+
+   +---------------+ <-- top/end of memory
+   |    256MB      |  debug server
+   +---------------+
+   |    256MB      |  MC
+   +---------------+
+   |     ...       |
+
+MC requires the memory to be aligned with 512MB, so even debug server is
+not enabled, 512MB is reserved, not 256MB.
+
 Flash Layout
 ============