PCI: iproc: Allow allocation of multiple MSIs

Add support for allocating multiple MSIs at the same time, so that the
MSI_FLAG_MULTI_PCI_MSI flag can be added to the msi_domain_info structure.

Avoid storing the hwirq in the low 5 bits of the message data, as it is
used by the device. Also fix an endianness problem by using readl().

Signed-off-by: Sandor Bodo-Merle <sbodomerle@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>