PCI: consolidate the PCI_DOMAINS and PCI_DOMAINS_GENERIC config options

Move the definitions to drivers/pci and let the architectures select
them.  Two small differences to before: PCI_DOMAINS_GENERIC now selects
PCI_DOMAINS, cutting down the churn for modern architectures.  As the
only architectured arm did previously also offer PCI_DOMAINS as a user
visible choice in addition to selecting it from the relevant configs,
this is gone now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>