arm64: dts: rockchip: introduce pclk_vio_grf in rk3399-eDP device node

The pclk_vio_grf supply power for VIO GRF IOs, if it is disabled,
driver would failed to operate the VIO GRF registers.

The clock is optional but one of the side effects of don't have this clk
is that the Samsung Chromebook Plus fails to recover display after a
suspend/resume with following errors:

    rockchip-dp ff970000.edp: Input stream clock not detected.
    rockchip-dp ff970000.edp: Timeout of video streamclk ok
    rockchip-dp ff970000.edp: unable to config video

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[this should also fix display failures when building rockchip-drm as module]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>