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