powerpc/xics: Rewrite XICS driver

This is a significant rework of the XICS driver, too significant to
conveniently break it up into a series of smaller patches to be honest.

The driver is moved to a more generic location to allow new platforms
to use it, and is broken up into separate ICP and ICS "backends". For
now we have the native and "hypervisor" ICP backends and one common
RTAS ICS backend.

The driver supports one ICP backend instanciation, and many ICS ones,
in order to accomodate future platforms with multiple possibly different
interrupt "sources" mechanisms.

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

arch/powerpc/include/asm/irq.h

@@ -142,6 +142,12 @@ extern struct irq_map_entry irq_map[NR_IRQS];
 
 extern irq_hw_number_t virq_to_hw(unsigned int virq);
 
+/* This will eventually -replace- virq_to_hw if/when we stash the
+ * HW number in the irq_data itself. We use a macro so we can inline
+ * it as irq_data isn't defined yet
+ */
+#define irq_data_to_hw(d) (irq_map[(d)->irq].hwirq)
+
 /**
  * irq_alloc_host - Allocate a new irq_host data structure
  * @of_node: optional device-tree node of the interrupt controller