mailbox: qcom-ipcc: flag IRQ NO_THREAD

PREEMPT_RT forces qcom-ipcc's handler to be threaded with interrupts
enabled, which triggers a warning in __handle_irq_event_percpu():
    irq 173 handler irq_default_primary_handler+0x0/0x10 enabled interrupts
    WARNING: CPU: 0 PID: 77 at kernel/irq/handle.c:161 __handle_irq_event_percpu+0x4c4/0x4d0

Mark it IRQF_NO_THREAD to avoid running the handler in a threaded
context with threadirqs or PREEMPT_RT enabled.

Signed-off-by: Eric Chanudet <echanude@redhat.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>