Documentation: dt-bindings: aspeed-pwm-tacho cooling device.

It's add support for cooling device creation to
aspeed-pwm-tacho.
Cooling device could be bound to a thermal zone
for the thermal control.

Signed-off-by: Mykola Kostenok <c_mykolak@mellanox.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>