watchdog: it87_wdt: Add support for IT8721F watchdog

This patch adds support for a watchdog in IT8721F Super IO
chip to it87_wdt driver.
This new chips differ from the older IT87xxxF chips 
in the following ways:

* WDT_GAMEPORT is not in IT8721F.
* WDT_PWROK is not in IT8721F.

Signed-off-by: Haruo Tomita <haruo.tomita@toshiba.co.jp>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>