genirq: provide irq_to_desc() to non-genirq architectures too

Impact: build fix on non-genirq architectures

Sam Ravnborg reported this build failure on sparc32 allmodconfig,
the GPIO drivers assume the presence of irq_to_desc():

 drivers/gpio/gpiolib.c: In function `gpiolib_dbg_show':
 drivers/gpio/gpiolib.c:1146: error: implicit declaration of function 'irq_to_desc'

Add it in the !genirq case too.
Reported-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Tested-by: NSam Ravnborg <sam@ravnborg.org>

include/linux/irqnr.h

@@ -8,7 +8,12 @@
 
 #ifndef CONFIG_GENERIC_HARDIRQS
 #include <asm/irq.h>
-# define nr_irqs		NR_IRQS
+
+/*
+ * Wrappers for non-genirq architectures:
+ */
+#define nr_irqs			NR_IRQS
+#define irq_to_desc(irq)	(&irq_desc[irq])
 
 # define for_each_irq_desc(irq, desc)		\
 	for (irq = 0; irq < nr_irqs; irq++)