IT8620E supports up to 6 pwm channels. Add support for it.
Also check if fan tachometers 4..6 are enabled before instantiating
the respective sysfs attributes.
Tested-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>