powerpc: Disable SPARSE_IRQ by default

The SPARSE_IRQ considerably adds overhead to critical path of IRQ
handling.  However it doesn't benefit much in space for most systems with
limited IRQ_NR.  Should be disabled unless really necessary.
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Kconfig

@@ -400,7 +400,7 @@ config IRQ_ALL_CPUS
 
 config SPARSE_IRQ
 	bool "Support sparse irq numbering"
-	default y
+	default n
 	help
 	  This enables support for sparse irqs. This is useful for distro
 	  kernels that want to define a high CONFIG_NR_CPUS value but still
@@ -409,7 +409,7 @@ config SPARSE_IRQ
 	  ( Sparse IRQs can also be beneficial on NUMA boxes, as they spread
 	    out the irq_desc[] array in a more NUMA-friendly way. )
 
-	  If you don't know what to do here, say Y.
+	  If you don't know what to do here, say N.
 
 config NUMA
 	bool "NUMA support"