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/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 61a8ffd..8ae2ce4 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -88,6 +88,8 @@
 # CONFIG_SPL_SIMPLE_BUS is not set
 CONFIG_ADC=y
 CONFIG_ADC_SANDBOX=y
+CONFIG_AXI=y
+CONFIG_AXI_SANDBOX=y
 CONFIG_CLK=y
 CONFIG_CPU=y
 CONFIG_DM_DEMO=y
@@ -202,3 +204,4 @@
 CONFIG_UNIT_TEST=y
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
+CONFIG_P2SB=y