dm: Tidy up 'dm tree' output when there are many devices

At present the 'Index' column assumes there is only one digit. But on some
devices (e.g. snow) there are a lot of regulators and GPIO banks. Adjust
the output to allow for two digits without messing up the display.

Also capatalise the heading to match.

Fixes: 5197dafc (dm: core: Widen the dump tree to show more of the
driver's name.)

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Liviu Dudau <liviu.dudau@foss.arm.com>