arm64: dts: rockchip: add the grf clk for dw-mipi-dsi on rk3399

The clk of grf must be enabled before writing grf
register for rk3399.

Signed-off-by: Nickey Yang <nickey.yang@rock-chips.com>
[the grf clock is already part of the binding since march 2017]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>