watchdog: it87_wdt: Add support for IT8728F watchdog.

This works the same way IT8721F works, but it supports WDT_PWROK
(checked on the datasheet).

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>