dm: test: Add tests for the generic PHY uclass

Those tests check:
- the ability for a phy-user to get a phy based on its name or its index
- the ability of a phy device (provider) to manage multiple ports
- the ability to perform operations on the phy (init,deinit,on,off)
- the behavior of the uclass when optional operations are not implemented

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/phy/Makefile

@@ -6,3 +6,4 @@
 #
 
 obj-$(CONFIG_$(SPL_)PHY) += phy-uclass.o
+obj-$(CONFIG_PHY_SANDBOX) += sandbox-phy.o