PCI: faraday: Convert IRQ masking to raw PCI config accessors

Current ftpci100 driver host bridge controller driver requires struct
pci_bus to be created in order to mask and clear IRQs using standard PCI
bus config accessors.

This struct pci_bus dependency is fictitious and burdens the driver with
unneeded constraints (eg to use separate APIs to create and scan the root
bus).

Add PCI raw config space accessors to PCIe ftpci100 driver and remove the
fictitious struct pci_bus dependency.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
[bhelgaas: folded in raw PCI read accessor from
http://lkml.kernel.org/r/20170621162651.25315-1-linus.walleij@linaro.org


The clock piece of the above posting goes with the separate "Add clock
handling" patch.]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>