ARM: dts: rockchip: Remove rockchip,grf from rk3288 tsadc

ARM: dts: rockchip: Remove rockchip,grf from rk3288 tsadc

According to rockchip-thermal.yaml, RK3288 does not require rockchip,grf,
so remove this invalid property.

This fixes the following dt-schema warning:

tsadc@ff280000 (rockchip,rk3288-tsadc): False schema does not allow [[53]]

The rockchip_thermal driver also confirms that grf is not needed as
the rk3288_tsadc_data contains:
 .grf_required = false,

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://patch.msgid.link/20260228013257.256973-2-festevam@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>