genirq: Move irq_chip_write_msi_msg() helper to core

No point to expose this to the world. The only legitimate user is the
core code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
(cherry picked from commit 74faaf7aa64c76b60db0f5c994fd43a46be772ce)
Signed-off-by: Alex Shi <alex.shi@linaro.org>