dm: core: Add a command to show driver model statistics

dm: core: Add a command to show driver model statistics

This command shows the memory used by driver model along with various
hints as to what it might be if some 'core' tags were moved to use the
tag list instead of a core (i.e. always-there) pointer.

This may help with future work to reduce memory usage.

Signed-off-by: Simon Glass <sjg@chromium.org>