irqchip: Add Kconfig menu

Add a menu for IRQ chip drivers. This makes the Device drivers menu be more
consistent (listing "subsystems" instead of specific options) and makes the
IRQCHIP options be listed in expected places for 'make menu|xconfig'.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: https://lkml.kernel.org/r/3db7385a-c6a1-5c93-0797-6f4b6b2b2cde@infradead.org

drivers/irqchip/Kconfig

+menu "IRQ chip support"
+
 config IRQCHIP
 	def_bool y
 	depends on OF_IRQ
@@ -321,3 +323,5 @@ config IRQ_UNIPHIER_AIDET
 	select IRQ_DOMAIN_HIERARCHY
 	help
 	  Support for the UniPhier AIDET (ARM Interrupt Detector).
+
+endmenu