dm: Add a demonstration/example driver

dm: Add a demonstration/example driver

As an example of how to write a uclass and a driver, provide a demo version
of each, accessible through the 'demo' command.

To use these with driver model, define CONFIG_CMD_DEMO and CONFIG_DM_DEMO.

The two demo drivers are enabled with CONFIG_DM_DEMO_SIMPLE and
CONFIG_DM_DEMO_SHAPE.

Signed-off-by: Simon Glass <[email protected]>
Signed-off-by: Marek Vasut <[email protected]>
Signed-off-by: Pavel Herrmann <[email protected]>
Signed-off-by: Viktor Křivák <[email protected]>
Signed-off-by: Tomas Hlavacek <[email protected]>