generic: sparse irqs: use irq_desc() together with dyn_array, instead of irq_desc[]

add CONFIG_HAVE_SPARSE_IRQ to for use condensed array.
Get rid of irq_desc[] array assumptions.

Preallocate 32 irq_desc, and irq_desc() will try to get more.

( No change in functionality is expected anywhere, except the odd build
  failure where we missed a code site or where a crossing commit itroduces
  new irq_desc[] usage. )

v2: according to Eric, change get_irq_desc() to irq_desc()

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/drivers/mfd/asic3.c b/drivers/mfd/asic3.c
index ba5aa20..e4c0db4 100644
--- a/drivers/mfd/asic3.c
+++ b/drivers/mfd/asic3.c
@@ -123,7 +123,7 @@
 					irqnr = asic->irq_base +
 						(ASIC3_GPIOS_PER_BANK * bank)
 						+ i;
-					desc = irq_desc + irqnr;
+					desc = irq_to_desc(irqnr);
 					desc->handle_irq(irqnr, desc);
 					if (asic->irq_bothedge[bank] & bit)
 						asic3_irq_flip_edge(asic, base,
@@ -136,7 +136,7 @@
 		for (i = ASIC3_NUM_GPIOS; i < ASIC3_NR_IRQS; i++) {
 			/* They start at bit 4 and go up */
 			if (status & (1 << (i - ASIC3_NUM_GPIOS + 4))) {
-				desc = irq_desc + asic->irq_base + i;
+				desc = irq_to_desc(asic->irq_base + i);
 				desc->handle_irq(asic->irq_base + i,
 						 desc);
 			}