test: dm: Add test for ECDSA UCLASS support

This test verifies that ECDSA_UCLASS is implemented, and that
ecdsa_verify() works as expected. The definition of "expected" is
"does not find a device, and returns -ENODEV".

The lack of a hardware-independent ECDSA implementation prevents us
from having one in the sandbox, for now.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>