powerpc/fsl_msi: add support for the fsl, msi property in PCI nodes

powerpc/fsl_msi: add support for the fsl, msi property in PCI nodes

On Freescale parts with multiple MSI controllers, the controllers are
combined into one "pool" of interrupts.  Whenever a device requests an MSI
interrupt, the next available interrupt from the pool is selected,
regardless of which MSI controller the interrupt is from.  This works
because each PCI bus has an ATMU to all of CCSR, so any PCI device can
access any MSI interrupt register.

The fsl,msi property is used to specify that a given PCI bus should only
use a specific MSI device.  This is necessary, for example, with the
Freescale hypervisor, because the MSI devices are assigned to specific
partitions.

Ideally, we'd like to be able to assign MSI devices to PCI busses within
the MSI or PCI layers.  However, there does not appear to be a mechanism
to do that.  Whenever the MSI layer wants to allocate an MSI interrupt to
a PCI device, it just calls arch_setup_msi_irqs().  It would be nice if we
could register an MSI device with a specific PCI bus.

So instead we remember the phandles of each MSI device, and we use that to
limit our search for an available interrupt.  Whenever we are asked to
allocate a new interrupt for a PCI device, we check the fsl,msi property
of the PCI bus for that device.  If it exists, then as we are looping over
all MSI devices, we skip the ones that don't have a matching phandle.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>