ARM: dts: rockchip: make rk3288-grf a simple-mfd

Similar to the pmu, the general register files contain a lot of different
setting bits grouped into general registers, but also some somewhat special
entities like the controls for some phy-blocks or the io-voltage control.
To be able to move these blocks under the grf node where they actually
belong, make it a simple-mfd.

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