dm: core: Add a uclass pre_probe() method for devices

Some uclasses want to set up a device before it is probed. Add a method
for this.

An example is with PCI, where a PCI uclass wants to set up its private
data for later use. This allows the device's uclass() method to make calls
whcih use that data (for example, read PCI memory regions from device
tree, set up bus numbers).

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