pci_ep: add pci endpoint sandbox driver

Add a dummy PCI endpoint for sandbox.
Supporting only a single function, it allows setting
and reading header configuration.

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/pci_endpoint/Makefile b/drivers/pci_endpoint/Makefile
index 0a849de..3cd9872 100644
--- a/drivers/pci_endpoint/Makefile
+++ b/drivers/pci_endpoint/Makefile
@@ -5,3 +5,4 @@
 
 obj-y += pci_ep-uclass.o
 obj-$(CONFIG_PCIE_CADENCE_EP) += pcie-cadence-ep.o
+obj-$(CONFIG_PCI_SANDBOX_EP) += sandbox-pci_ep.o