PCI: xgene: Use pci_host_probe() to register host

The xgene host driver does the same host registration and bus scanning
calls as pci_host_probe, so let's use it instead.

Link: https://lore.kernel.org/r/20200522234832.954484-10-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Toan Le <toan@os.amperecomputing.com>