Add a test for the dm drivers command. Also fix a null pointer dereference
revealed by said test.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

This adds a subcommand to dm to dump out what drivers are installed, and their
compatible strings. I have found this useful in ensuring that I have the correct
drivers compiled, and that I have put in the correct compatible strings.

Signed-off-by Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

The "dm" command under CONFIG_CMD_DM should live under cmd/ rather than
test/dm/ so move it.

Signed-off-by: Tom Rini <trini@konsulko.com>