sandbox: Add a test device that uses of-platdata

Start up the test devices. These print out of-platdata contents, providing a
check that the of-platdata feature is working correctly.

The device-tree changes are made to sandbox.dts rather than test.dts. since
the former controls the of-platdata generation.

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