PCI: Also call .add_bus() callback for root bus

Similar to pcibios_add_bus(), call pci_ops.add_bus() when the root bus
is added. This allows host bridge drivers to do any setup requiring a
bus pointer.

There are currently no .add_bus() callbacks, so this is safe to do.

Link: https://lore.kernel.org/r/20200821035420.380495-15-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>