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: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>