dt-bindings: thermal: Fix missing required property

When the thermal zone description was converted to yaml schema, the
required 'trips' property was forgotten.

The initial text bindings was describing:

"
[ ... ]

* Thermal zone nodes

The thermal zone node is the node containing all the required info
for describing a thermal zone, including its cooling device bindings. The
thermal zone node must contain, apart from its own properties, one sub-node
containing trip nodes and one sub-node containing all the zone cooling maps.

Required properties:
- polling-delay:        The maximum number of milliseconds to wait between polls
  Type: unsigned        when checking this thermal zone.
  Size: one cell

- polling-delay-passive: The maximum number of milliseconds to wait
  Type: unsigned        between polls when performing passive cooling.
  Size: one cell

- thermal-sensors:      A list of thermal sensor phandles and sensor specifier
  Type: list of         used while monitoring the thermal zone.
  phandles + sensor
  specifier

- trips:                A sub-node which is a container of only trip point nodes
  Type: sub-node        required to describe the thermal zone.

Optional property:
- cooling-maps:         A sub-node which is a container of only cooling device
  Type: sub-node        map nodes, used to describe the relation between trips
                        and cooling devices.
  [ ... ]

"

Now the schema describes:

"
    [ ... ]

    required:
      - polling-delay
      - polling-delay-passive
      - thermal-sensors

    [ ... ]
"

Add the missing 'trips' property in the required properties.

Fixed: 1202a442 ("dt-bindings: thermal: Add yaml bindings for thermal zones")
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220809085629.509116-3-daniel.lezcano@linaro.org