ARM: PCI: provide a default bus scan implementation

Most PCI implementations perform simple root bus scanning.  Rather than
having each group of platforms provide a duplicated bus scan function,
provide the PCI configuration ops structure via the hw_pci structure,
and call the root bus scanning function from core ARM PCI code.

Acked-by: Krzysztof Hałasa <khc@pm.waw.pl>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>