dt-bindings: thermal: make cooling-maps property optional

Cooling-maps doesn't have to be a required property because there may
be no cooling device on system, or there are no enough cooling devices for
each thermal zone in multiple thermal zone cases since cooling devices
can't be shared.
So make this property optional to remove such limitations.

For thermal zones with no cooling-maps, there could be critic trips
that can trigger CPU reset or shutdown. So they still can take actions.

Signed-off-by: Yuantian Tang <andy.tang@nxp.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200309045411.21859-1-andy.tang@nxp.com