cmd: mem: Drop #ifdef for MEM_SUPPORT_64BIT_DATA

This is defined only when __lp64__ is defined. That means that ulong is
64 bits long. Therefore we don't need to use a separate u64 type on those
architectures.

Fix up the code to take advantage of that, removing the preprocessor
conditions.

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