We may currently lose interrupts during SPE context switch, as we alter
the INT_Route register. Because the IIC uses a per-thread priority
status, changing the interrupt routing to a different thread means that
the IRQ is no longer masked by the priority status, so we end up with
two fasteoi IRQ handlers executing for the one irq_desc. The fasteoi
handler doesn't handle multiple IRQs, so drops the second one.

Fix this by using our own flow handler. This is based on
handle_edge_irq, but issues an eoi after IRQs are handled, and doesn't
do any mask/unmasking.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>