dt-bindings: rockchip-thermal: fix the misleading description

"rockchip,hw-tshut-temp", "rockchip,hw-tshut-mode" and
"rockchip,hw-tshut-polarity" are not a required properties
actually as the code could also work by loading the default
settings there. So it is apprently misleading, although we
prefer to get these from DT. And it seems we miss the 'rockchip,grf'
here which should also be an optional property.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>