x86: Support a fake PCI device with of-platdata-inst

With TPL we don't need full PCI support and it adds to code size. Instead,
a simple_bus driver is good enough to be able to read and write the PCI
config and do a little basic setup.

So at present there are two drivers in U-Boot called pci_x86. One is in
UCLASS_PCI, used in SPL and U-Boot proper. The other is in
UCLASS_SIMPLE_BUS and used only in TPL.

Add a tag to tell dtoc about this, so it knows which one to use when
generating the devices and uclasses.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>