dm: pci: Avoid a driver model build error with CONFIG_CMD_PCI_ENUM

dm: pci: Avoid a driver model build error with CONFIG_CMD_PCI_ENUM

This is not supported with driver model, so print a message instead of
generating a build error. Rescanning PCI is not yet implemented.

This function will be implemented later once some additional PCI driver
model improvements are merged. It was confirmed on the mailing list
that no one on the tegra side will miss this feature, so it is disabled
for tegra.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>