[WATCHDOG] Add support for WM831x watchdog

The WM831x series of devices provide a watchdog with configurable
behaviour on timer expiry.

Currently this driver support refreshes via a register or GPIO line and
autonomous refreshes from a hardware source (eg, a clock).

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>