dm: Change CMD_DM enabling

CMD_DM is used for debug purpose and it shouldn't be enabled by default
via Kconfig. Unfortunately this is in the tree for quite a long time
that's why solution is to use imply DM for all targets which are
enabling DM.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>