arm64: dts: rockchip: add missing rockchip,grf property to rk356x

This commit fixes the error messages

  rockchip_clk_register_muxgrf: regmap not available
  rockchip_clk_register_branches: failed to register clock clk_ddr1x: -524

during boot by providing the missing rockchip,grf property.

Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Tested-by: Peter Geis <pgwipeout@gmail.com>
Link: https://lore.kernel.org/r/20210823123911.12095-2-michael.riesch@wolfvision.net


Signed-off-by: Heiko Stuebner <heiko@sntech.de>