test: Add tests for CPU uclass

Add a sandbox CPU driver, and some tests for the CPU uclass.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 3f5a634..8b1ba91 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -47,4 +47,5 @@
 obj-$(CONFIG_AXI) += axi.o
 obj-$(CONFIG_MISC) += misc.o
 obj-$(CONFIG_DM_SERIAL) += serial.o
+obj-$(CONFIG_CPU) += cpu.o
 endif