watchdog: ziirave_wdt: Rename "trigger" reset reason "hw watchdog"

The Zodiac watchdog is implemented on a microcontoller. The reset reason
currently labelled "trigger" is not to detect when the watchdog has
triggered (as had been initially understood and suggested by the naming),
but to inform the reader that the watchdog, which in fact has it's own
hardware watchdog, has been reset because the hardware watchdog has
triggered. Renaming to "hw watchdog".

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>