README: drop mention of MEM_SUPPORT_64BIT_DATA

The first sentence is half-way true; the macro is always defined, but
has the value 0 or 1.

The second is outright false. A lot of code guarded by
MEM_SUPPORT_64BIT_DATA uses a "ulong" to store values, so if
sizeof(long) is not 8, that code would probably compile, but not work
at all as expected.

It would probably be possible to make all such code explicitly use u64
and thus make it work on 32 bit targets, but until that is done, do
not pretend that it's ok to override the automatic value of
MEM_SUPPORT_64BIT_DATA.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

README

@@ -1240,9 +1240,6 @@ typically in board_init_f() and board_init_r().
 Configuration Settings:
 -----------------------
 
-- 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;
 		undefine this when you're short of memory.