powerpc/xics: Add ICP OPAL backend

This adds a new XICS backend that uses OPAL calls, which can be
used when we don't have native support for the platform interrupt
controller.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/sysdev/xics/xics-common.c

@@ -404,8 +404,11 @@ void __init xics_init(void)
 	/* Fist locate ICP */
 	if (firmware_has_feature(FW_FEATURE_LPAR))
 		rc = icp_hv_init();
-	if (rc < 0)
+	if (rc < 0) {
 		rc = icp_native_init();
+		if (rc == -ENODEV)
+		    rc = icp_opal_init();
+	}
 	if (rc < 0) {
 		pr_warning("XICS: Cannot find a Presentation Controller !\n");
 		return;