common: Move and rename CONFIG_SYS_SUPPORT_64BIT_DATA

This is not really a CONFIG since it is not intended to be set by boards.
Move it into the compiler header with other similar defines, and rename
it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 78f59c3..c1324c1 100644
--- a/README
+++ b/README
@@ -2460,7 +2460,7 @@
 Configuration Settings:
 -----------------------
 
-- CONFIG_SYS_SUPPORT_64BIT_DATA: Defined automatically if compiled as 64-bit.
+- MEM_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;