PCI: altera-msi: Allow building as module

Altera MSI IP is a soft IP and is only available after
an FPGA image (with design containing it) is programmed.

Make driver modulable to support use case FPGA image is programmed the
after kernel has booted, so that the driver can be loaded upon request.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>