PCI: host-generic: Support building as modules

PCI: host-generic: Support building as modules

Enable building host-generic and its host-common dependency as a
module.

Link: https://lore.kernel.org/r/20200409234923.21598-3-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Andrew Murray <amurray@thegoodpenguin.co.uk>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-pci@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org