realtek: use higher priority for timer interrupts

realtek: use higher priority for timer interrupts

The assigned output index for the event timers was quite low, lower even
than the ethernet interrupt. This means that high network load could
preempt timer interrupts, possibly leading to all sorts of strange
behaviour.

Increase the interrupt output index of the event timers to 5, which is
the highest priority output and corresponds to the (otherwise unused)
MIPS CPU timer interrupt.

Fixes: a75b9e3ecb61 ("realtek: Adding RTL930X sub-target")
Signed-off-by: Sander Vanheule <sander@svanheule.net>