dm: Add a test for of-platdata

dm: Add a test for of-platdata

Add a simple test which checks that the of-platdata system is working
correctly. The sequence is as follows:

- SPL starts up and probes all the UCLASS_MISC drivers
- There are 3 of these in sandbox.dts
- Therefore there should be 3 U_BOOT_DEVICE() declarations in dt-platdata.c
- These should produce 3 sandbox_spl_test devices
- Each device prints out its platform data when probed
- This test checks for this output and compares it against expectations

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