Add 64-bit data support for memory commands

Add 64-bit data for memory commands, such as md, mw, mm, cmp. The new
size ".q " is introduced.

For 64-bit architecture, 64-bit data is enabled by default, by detecting
compiler __LP64__. It is optional for other architectures.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/README b/README
index ff49260..216f0c7 100644
--- a/README
+++ b/README
@@ -3470,6 +3470,9 @@
 Configuration Settings:
 -----------------------
 
+- CONFIG_SYS_SUPPORT_64BIT_DATA: Defined automatically if compiled as 64-bit.
+		Optionally it can be defined to support 64-bit memory commands.
+
 - CONFIG_SYS_LONGHELP: Defined when you want long help messages included;
 		undefine this when you're short of memory.