sandbox: Add PCI driver and test for p2sb

Add a sandbox driver and PCI-device emulator for p2sb. Also add a test
which uses a simple 'adder' driver to test the p2sb functionality.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/test/dm/Makefile b/test/dm/Makefile
index 10a19a0..129ccb3 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -32,6 +32,7 @@
 obj-$(CONFIG_OSD) += osd.o
 obj-$(CONFIG_DM_VIDEO) += panel.o
 obj-$(CONFIG_DM_PCI) += pci.o
+obj-$(CONFIG_P2SB) += p2sb.o
 obj-$(CONFIG_PCI_ENDPOINT) += pci_ep.o
 obj-$(CONFIG_PCH) += pch.o
 obj-$(CONFIG_PHY) += phy.o