arm: dts: rockchip: rk3288: move thermal sub nodes to dtsi

In order to better compare the Linux rk3288.dtsi version
with the u-boot version move thermal sub nodes to the dtsi
file and remove rk3288-thermal.dtsi

Changed:
  replace underscore in nodename
  remove comments about sensor and ID
  use gpu phandle
  add #cooling-cells to gpu node
  lower critical temparature
  remove linux,hwmon property

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>