dm: pci: Add a driver-model version of pci_find_class()

Add a function which scans the driver model device information rather
than scanning the PCI bus again.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>