powerpc: Make SPARSE_IRQ required

All IRQs on powerpc are managed via irq_domain anyway, there isn't really
any advantage to turning SPARSE_IRQ off, and it's the direction we want
to take the kernel design anyway.  This patch makes powerpc always use
SPARSE_IRQ.

On pseries_defconfig, SPARSE_IRQ adds only about 0x300 bytes to the
.text sections, and removes about 0x20000 from the data section for the
static irq_desc table.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Ben Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/Kconfig

@@ -134,6 +134,7 @@ config PPC
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS && PPC_BOOK3S_64
 	select HAVE_GENERIC_HARDIRQS
 	select HAVE_SPARSE_IRQ
+	select SPARSE_IRQ
 	select IRQ_PER_CPU
 	select GENERIC_IRQ_SHOW
 	select GENERIC_IRQ_SHOW_LEVEL

arch/powerpc/kernel/irq.c

@@ -93,10 +93,6 @@ extern int tau_interrupts(int);
 
 #ifdef CONFIG_PPC64
 
-#ifndef CONFIG_SPARSE_IRQ
-EXPORT_SYMBOL(irq_desc);
-#endif
-
 int distribute_irqs = 1;
 
 static inline notrace unsigned long get_hard_enabled(void)