irq_domain: Move irq_domain code from powerpc to kernel/irq

This patch only moves the code.  It doesn't make any changes, and the
code is still only compiled for powerpc.  Follow-on patches will generalize
the code for other architectures.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Milton Miller <miltonm@bga.com>
Tested-by: Olof Johansson <olof@lixom.net>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 1919634..303703d 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -135,6 +135,7 @@
 	select HAVE_GENERIC_HARDIRQS
 	select HAVE_SPARSE_IRQ
 	select IRQ_PER_CPU
+	select IRQ_DOMAIN
 	select GENERIC_IRQ_SHOW
 	select GENERIC_IRQ_SHOW_LEVEL
 	select IRQ_FORCED_THREADING