ARM: only include mach/irqs.h for !SPARSE_IRQ

Make mach/irqs.h optional for SPARSE_IRQ. With this change mach/irqs.h can
be removed by converting platforms over to sparse irq.

Platforms either need to set nr_irqs in their machine desc or all irqchips
used by a platform need to allocate their irq_descs. There cannot be a
mixture. Once this is done, the platforms can select SPARSE_IRQ. shmobile
does the latter, and mmp and pxa do the former.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

arch/arm/include/asm/irq.h

 #ifndef __ASM_ARM_IRQ_H
 #define __ASM_ARM_IRQ_H
 
+#define NR_IRQS_LEGACY	16
+
+#ifndef CONFIG_SPARSE_IRQ
 #include <mach/irqs.h>
+#else
+#define NR_IRQS NR_IRQS_LEGACY
+#endif
 
 #ifndef irq_canonicalize
 #define irq_canonicalize(i)	(i)
 #endif
 
-#define NR_IRQS_LEGACY	16
-
 /*
  * Use this value to indicate lack of interrupt
  * capability