irqchip: fill in empty Kconfig

Commit 89214f00 "ARM: bcm2835: add interrupt controller driver" added an
empty drivers/irqchip/Kconfig. Empty files apparently don't work well
with git (sometimes, with some versions?) so add some dummy content to
resolve this issue.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Olof Johansson <olof@lixom.net>