dm: Add basic tests

Add some tests of driver model functionality. Coverage includes:

- basic init
- binding of drivers to devices using platform_data
- automatic probing of devices when referenced
- availability of platform data to devices
- lifecycle from bind to probe to remove to unbind
- renumbering within a uclass when devices are probed/removed
- calling driver-defined operations
- deactivation of drivers when removed
- memory leak across creation and destruction of drivers/uclasses
- uclass init/destroy methods
- automatic probe/remove of children/parents when needed

This function is enabled for sandbox, using CONFIG_DM_TEST.

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