irqchip: add to the directories part of the IRQ subsystem in MAINTAINERS

Now that the drivers/irqchip/ directory is getting more code, it needs
a maintainer. The obvious maintainer for it is Thomas Gleixner, who is
maintaining the overall IRQ subsystem. So we add drivers/irqchip/ in
the list of directories that are part of the IRQ subsystem.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Rob Herring <rob.herring@calxeda.com>