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_defconfig b/configs/sandbox_defconfig
index 64245f7..36a1dfa 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -132,6 +132,7 @@
 CONFIG_CROS_EC_LPC=y
 CONFIG_CROS_EC_SANDBOX=y
 CONFIG_CROS_EC_SPI=y
+CONFIG_P2SB=y
 CONFIG_PWRSEQ=y
 CONFIG_SPL_PWRSEQ=y
 CONFIG_I2C_EEPROM=y