Extend simple_mfd driver to expose a syscon interface

[ganbold]

Summary

Extend simple_mfd driver to expose a syscon interface if
that node is also compatible with syscon. For instance,
Rockchip RK3399's GRF (General Register Files) is compatible
with simple-mfd as well as syscon and has devices like
usb2-phy, emmc-phy and pcie-phy etc. under it.

[manu]

sys/dev/fdt/simple_mfd.c

40

you need sys/mutex.h too otherwise it doesn't compile

48

#include <dev/fdt/simple_mfd.h>

155

The rid variable isn't needed

[ganbold]

Updated.
Used #include <dev/fdt/simple_mfd.h>
It compiles and working fine without sys/mutex.h (for instance sys/dev/extres/syscon/* files doesn't use it)
rid var is needed for bus_alloc_resource_any()

[ganbold]

Updated.
Added sys/mutex.h