Add a flag field to struct gic_irqsrc and use it to mark when we should
write to the End of Interrupt (EOI) register before handling the interrupt.
This should be a noop as it will be set for all edge triggered interrupts,
however this will not be the case for MSI interrupts. These are also edge
triggered, however we should not write to the EOI register until later in
arm_gic_pre_ithread.

Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation