test: dm: pci: Test more than one PCI host controller

So far there is only one PCI host controller in the sandbox test
configuration. This is normally the case for x86, but it can be
common on other architectures like ARM/PPC to have more than one
PCI host controller in the system.

This updates the case to cover such scenario.

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