powerpc/powernv: Add OPAL ICS backend

OPAL handles HW access to the various ICS or equivalent chips
for us (with the exception of p5ioc2 based HEA which uses a

different backend) similarily to what RTAS does on pSeries.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/xics.h

@@ -27,10 +27,18 @@
 #define MAX_NUM_PRIORITIES	3
 
 /* Native ICP */
+#ifdef CONFIG_PPC_ICP_NATIVE
 extern int icp_native_init(void);
+#else
+static inline int icp_native_init(void) { return -ENODEV; }
+#endif
 
 /* PAPR ICP */
+#ifdef CONFIG_PPC_ICP_HV
 extern int icp_hv_init(void);
+#else
+static inline int icp_hv_init(void) { return -ENODEV; }
+#endif
 
 /* ICP ops */
 struct icp_ops {
@@ -51,7 +59,18 @@ extern const struct icp_ops *icp_ops;
 extern int ics_native_init(void);
 
 /* RTAS ICS */
+#ifdef CONFIG_PPC_ICS_RTAS
 extern int ics_rtas_init(void);
+#else
+static inline int ics_rtas_init(void) { return -ENODEV; }
+#endif
+
+/* HAL ICS */
+#ifdef CONFIG_PPC_POWERNV
+extern int ics_opal_init(void);
+#else
+static inline int ics_opal_init(void) { return -ENODEV; }
+#endif
 
 /* ICS instance, hooked up to chip_data of an irq */
 struct ics {