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 <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>

drivers/pci/controller/Kconfig

···
 	  FPGA.
 
 config PCIE_ALTERA_MSI
-	bool "Altera PCIe MSI feature"
+	tristate "Altera PCIe MSI feature"
 	depends on PCIE_ALTERA
 	depends on PCI_MSI_IRQ_DOMAIN
 	help

drivers/pci/controller/pcie-altera-msi.c

···
 #include <linux/interrupt.h>
 #include <linux/irqchip/chained_irq.h>
 #include <linux/init.h>
+#include <linux/module.h>
 #include <linux/msi.h>
 #include <linux/of_address.h>
 #include <linux/of_irq.h>
···
 {
 	return platform_driver_register(&altera_msi_driver);
 }
+
+static void __exit altera_msi_exit(void)
+{
+	platform_driver_unregister(&altera_msi_driver);
+}
+
 subsys_initcall(altera_msi_init);
+MODULE_DEVICE_TABLE(of, altera_msi_of_match);
+module_exit(altera_msi_exit);
+MODULE_LICENSE("GPL v2");