hwmon: (it87) Add support for all pwm channels on IT8620E

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: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>