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 <sjg@chromium.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Pavel Herrmann <morpheus.ibis@gmail.com>
Signed-off-by: Viktor Křivák <viktor.krivak@gmail.com>
Signed-off-by: Tomas Hlavacek <tmshlvck@gmail.com>