x86: apl: Avoid accessing the PCI bus before it is probed

x86: apl: Avoid accessing the PCI bus before it is probed

The PCI bus is not actually probed by the time the ofdata_to_platdata()
method is called since that happens in the uclass's post_probe() method.
Update the PMC and P2SB drivers to access the bus in its probe() method.

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