arm64: dts: rockchip: make rk3368 grf syscons simple-mfds

The general register files do contain a lot of separate functions and
while some really are only registers with a lot of different 1-bit
settings, there are also a lot of them containing some bigger function
blocks. To be able to define these as sub-devices, make them simple-mfds.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: David Wu <david.wu@rock-chips.com>