dm: test: Add test for device removal

Add a test for the correct device removal. Currently two different ways
for device removal are supported:

- Normal device removal via the device_remove() API
- Removal via selective device driver flags (DM_FLAG_ACTIVE_DMA)

This new test "remove_active_dma" adds tests cases for those both ways
of removal. This is done by adding a new test driver, which has this
flag set.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>