watchdog-for-linus-v4.16

- Converted to use watchdog subsystem: sp5100_tco, i6300esb, xen_wdt
- New drivers: Spreadtrum, Realtek RTD1295
- Added r8a77970 support to renesas-wdt
- Added jz4780 support to jz4740
- Removed at32ap700x_wdt driver
- Removed obsolete documentation
- Various bug fixes in watchdog core
- Cleanup and bug fixes in several drivers
- Added Guenter Roeck as co-maintainer