watchdog: orion: Handle the interrupt so it's properly acked

DT-enabled plaforms, where the irqchip driver for the brigde interrupt
controller is available, can handle the watchdog IRQ properly. Therefore,
request the interrupt and add a dummy handler that merely calls panic().

This is done in order to have an initial 'ack' of the interruption,
which clears the watchdog state.

Furthermore, since some platforms don't have such IRQ, this commit
makes the interrupt specification optional.

Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Tested-By: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>