dt-bindings: brcm,avs-ro-thermal: Fix binding check issues

Drop the reg property since this only necessary for the parent and
add the missing thermal-sensor-cells property description.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: 37e9cc08e9678 ("dt-bindings: Add Broadcom AVS RO thermal")
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1579200077-17496-1-git-send-email-stefan.wahren@i2se.com