MAINTAINERS: Appoint Jiang and Marc as irqdomain maintainers

Ben was pretty surprised that he is still listed as the maintainer and
he has no objections against transferring the duty to those who
rumaged in and revamped that code in the recent past.

Add kernel/irq/msi.c to the affected files as it's part of the shiny
new hierarchical irqdomain machinery.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Grant Likely <grant.likely@linaro.org>