arm64: dts: rockchip: make rk3399's grf a "simple-mfd"

Per the examples in
Documentation/devicetree/bindings/phy/rockchip-emmc-phy.txt, we need the
grf node to be a simple-mfd in order to properly enumerate child devices
like our eMMC PHY.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>

[directly mimic for the pmugrf, which will need the same change later
and there is no need to pollute commit history with another patch]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>