dm: fix alignment of help message of "dm" command

Currently, "help dm" shows as follows:

=> help dm
dm - Driver model low level access

Usage:
dm tree         Dump driver model tree ('*' = activated)
dm uclass        Dump list of instances for each uclass
dm devres        Dump list of device resources for each device

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>