irqchip: orion: clear bridge cause register on init

It is good practice to mask and clear pending irqs on init. We already
mask all irqs, so also clear the bridge irq cause register.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: <stable@vger.kernel.org> # v3.10+
Signed-off-by: Jason Cooper <jason@lakedaemon.net>