axi: Add AXI sandbox driver and simple emulator

Add test infrastructure and tests for the AXI uclass.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/drivers/axi/Makefile b/drivers/axi/Makefile
index 18d9380..66b6c5a 100644
--- a/drivers/axi/Makefile
+++ b/drivers/axi/Makefile
@@ -7,3 +7,6 @@
 
 obj-$(CONFIG_AXI) += axi-uclass.o
 obj-$(CONFIG_IHS_AXI) += ihs_axi.o
+obj-$(CONFIG_SANDBOX) += axi-emul-uclass.o
+obj-$(CONFIG_SANDBOX) += sandbox_store.o
+obj-$(CONFIG_AXI_SANDBOX) += axi_sandbox.o