ARM: dts: rockchip: add grf reference in rk3288 tsadc node

The following message can be seen during boot:

  rockchip-thermal ff280000.tsadc: Missing rockchip,grf property

Fix this by adding rockchip,grf property to tsadc node.

The warning itself is not relevant on rk3288 right now, as the
tsadc doesn't need to set GRF-values at this point and only newer
variants do.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>