cmd_mem: fix cp command

The "cp" command has not worked since
commit 0628ab8e,
because of the following lines, which set the destination
and the source to the same address.

	buf = map_sysmem(addr, bytes);
	src = map_sysmem(addr, bytes);

Tested-by: Tom Rini <trini@ti.com>
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>