dm_test: improve the appearance shown by "dm tree" command
The command "dm tree" lists devices in a tree-like format.
This commit makes it look more like what the Unix command "tree"
shows.
=> dm tree
Class Probed Name
----------------------------------------
root [ + ] root_driver
demo [ ] |-- demo_shape_drv
demo [ ] |-- demo_simple_drv
demo [ ] |-- demo_shape_drv
demo [ ] |-- demo_simple_drv
demo [ ] |-- demo_shape_drv
test [ ] |-- test_drv
test [ ] |-- test_drv
test [ ] |-- test_drv
gpio [ ] |-- gpio_sandbox
serial [ ] |-- serial_sandbox
serial [ + ] |-- serial
demo [ ] |-- triangle
demo [ ] |-- square
demo [ ] |-- hexagon
gpio [ ] |-- gpios
spi [ ] |-- spi@0
spi_emul [ ] | `-- flash@0
cros_ec [ + ] `-- cros-ec@0
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>