irqchip: orion: clear stale interrupts in irq_startup

Bridge IRQ_CAUSE bits are asserted regardless of the corresponding bit in
IRQ_MASK register. To avoid interrupt events on stale irqs, we have to clear
them before unmask. This installs an .irq_startup callback to ensure stale
irqs are cleared before initial unmask.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: <stable@vger.kernel.org>: f56c0738b5c2: "irqchip: orion: clear bridge cause register on init"
Cc: <stable@vger.kernel.org>: 38bd80b84fca: "irqchip: orion: use handle_edge_irq on bridge irqs"
Cc: <stable@vger.kernel.org> # v3.10+
Signed-off-by: Jason Cooper <jason@lakedaemon.net>