Update MEM_SUPPORT_64BIT_DATA to be always defined

Define this macro always so we don't need the preprocessor to check it.
Convert the users to #if instead of #ifdef.

Note that '#if MEM_SUPPORT_64BIT_DATA' does not give an error if the
macro is not define. It just assumes zero.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>