ARM/PCI, designware, xilinx: Use pci_scan_root_bus_msi()

ARM/PCI, designware, xilinx: Use pci_scan_root_bus_msi()

ARM previously stored the msi_controller pointer in its sysdata, struct
pci_sys_data, and implemented pcibios_msi_controller() to retrieve it.
That made PCI host controller drivers specific to ARM because they had to
put the msi_controller pointer in the ARM-specific pci_sys_data.

There is now a generic mechanism, pci_scan_root_bus_msi(), for giving the
msi_controller pointer to the PCI core.  Use this for all ARM systems and
for the DesignWare and Xilinx PCI host controller drivers.

This removes an ARM dependency from the DesignWare, DRA7xx, EXYNOS, i.MX6,
Keystone, Layerscape, SPEAr13xx, and Xilinx drivers.

[bhelgaas: changelog, split into separate patch]
Suggested-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
CC: Pratyush Anand <pratyush.anand@gmail.com>
CC: Arnd Bergmann <arnd@arndb.de>
CC: Simon Horman <horms@verge.net.au>
CC: Russell King <linux@arm.linux.org.uk>
CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
CC: Thierry Reding <thierry.reding@gmail.com>
CC: Michal Simek <michal.simek@xilinx.com>
CC: Marc Zyngier <marc.zyngier@arm.com>