pci: Add support for p2sb uclass

The Primary-to-Sideband bus (P2SB) is used to access various peripherals
through memory-mapped I/O in a large chunk of PCI space. The space is
segmented into different channels and peripherals are accessed by
device-specific means within those channels. Devices should be added in
the device tree as subnodes of the p2sb.

This adds a uclass and enables it for sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

include/dm/uclass-id.h

@@ -70,6 +70,7 @@ enum uclass_id {
 	UCLASS_NOP,		/* No-op devices */
 	UCLASS_NORTHBRIDGE,	/* Intel Northbridge / SDRAM controller */
 	UCLASS_NVME,		/* NVM Express device */
+	UCLASS_P2SB,		/* (x86) Primary-to-Sideband Bus */
 	UCLASS_PANEL,		/* Display panel, such as an LCD */
 	UCLASS_PANEL_BACKLIGHT,	/* Backlight controller for panel */
 	UCLASS_PCH,		/* x86 platform controller hub */