arm64: PCI: Restructure pcibios_root_bridge_prepare()

Restructure the pcibios_root_bridge_prepare() as the preparation for
supporting cases when no real ACPI device is related to the PCI host
bridge.

No functional change.

Link: https://lore.kernel.org/r/20210726180657.142727-4-boqun.feng@gmail.com


Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>