cmd: mem: Use memcpy for 'cp' command

Simplify the 'cp' command implementation by using the memcpy() function,
which brings the additional benefit of performance gain for those who have
CONFIG_USE_ARCH_MEMCPY selected.

Tested on a mx6qsabreauto board where a 5x gain in performance is seen
when reading 10MB from the parallel NOR memory.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>