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/Kconfig b/drivers/axi/Kconfig
index ae80c98..f81d843 100644
--- a/drivers/axi/Kconfig
+++ b/drivers/axi/Kconfig
@@ -22,4 +22,11 @@
 	  Interface (IHS AXI) bus on a gdsys IHS FPGA used to communicate with
 	  IP cores in the FPGA (e.g. video transmitter cores).
 
+config AXI_SANDBOX
+	bool "Enable AXI sandbox driver"
+	depends on DM
+	help
+	  Support AXI (Advanced eXtensible Interface) emulation for the sandbox
+	  environment.
+
 endif