[PATCHv2] watchdog: rti-wdt: support attaching to running wdt

[Tero Kristo]

Hi,

Version 2 of the series has quite a few changes based on feedback on
previous version.

1) Add new watchdog core API for adjusting the last hw keepalive time
   (Patch #2)
2) Modify the driver to support adjusting the window size, current
   driver only supports 50% window size. The window size is configured
   via a module parameter.
3) Modify the attach mechanism to running wdt to configure the heartbeat
   and window size based on running HW wdt setup.
4) Fix a runtime PM issue that was noticed while dealing with the new
   module parameter (tested via rmmod / modprobe)

-Tero


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