At present devres.h is included in all files that include dm.h but few
make use of it. Also this pulls in linux/compat which adds several more
headers. Drop the automatic inclusion and require files to include devres
themselves. This provides a good indication of which files use devres.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>

The devres functionality has very few users in U-Boot, but it still should
have tests. Add a few basic tests of the main functions.

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

At present these functions are lumped in with the core device functions.
They have their own #ifdef to control their availability, so it seems
better to split them out.

Move them into their own header file.

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