test: dm: add MDIO test

A very simple test for DM_MDIO, mimicks a register write/read through the
sandbox bus to a dummy PHY.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 420bf81..6a36cc0 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -62,4 +62,5 @@
 obj-$(CONFIG_TEE) += tee.o
 obj-$(CONFIG_VIRTIO_SANDBOX) += virtio.o
 obj-$(CONFIG_DMA) += dma.o
+obj-$(CONFIG_DM_MDIO) += mdio.o
 endif