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/drivers/cpu/Makefile b/drivers/cpu/Makefile
index db515f6..980c68f 100644
--- a/drivers/cpu/Makefile
+++ b/drivers/cpu/Makefile
@@ -7,3 +7,4 @@
 obj-$(CONFIG_CPU) += cpu-uclass.o
 
 obj-$(CONFIG_ARCH_BMIPS) += bmips_cpu.o
+obj-$(CONFIG_SANDBOX) += cpu_sandbox.o