watchdog: it87_wdt: add IT8620E ID

This adds IT8620E chip ID to it87_wdt driver.
Such chip is often found on current Gigabyte motherboards, it is allegedly
custom made for this manufacturer.

Upon testing it looks like it has a 16-bit timer and cannot be reset via
game port (only via CIR), so it is similar to IT87{18,20,21,28,83} chips.

Tested on GA-F2A88XM-HD3P board.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>