Migrate dm command to use U_BOOT_CMD_WITH_SUBCMDS() helper macro, to reduce
duplicated code. We can also drop the CONFIG_NEEDS_MANUAL_RELOC exception,
as the command list is updated post relocation in board_r.c initcall
initr_manual_reloc_cmdtable().
Signed-off-by: Ovidiu Panait <ovpanait@gmail.com>