sandbox: scmi: test against a single scmi agent

As per DT bindings since Linux kernel v5.14, the device tree can define
only 1 SCMI agent node that is named scmi [1]. As a consequence, change
implementation of the SCMI driver test through sandbox architecture to
reflect that.

This change updates sandbox test DT and sandbox SCMI driver accordingly
since all these are impacted.

Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>