test: cpu: add test for release CPU core.

Add test for API cpu_release_core().

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/cpu.c b/test/dm/cpu.c
index acba810..8af2531 100644
--- a/test/dm/cpu.c
+++ b/test/dm/cpu.c
@@ -43,6 +43,8 @@
 	ut_assertok(cpu_get_vendor(dev, text, sizeof(text)));
 	ut_assertok(strcmp(text, "Languid Example Garbage Inc."));
 
+	ut_assertok(cpu_release_core(dev, 0));
+
 	return 0;
 }