FreeBSD/src 7f8c4c7 (r351184)sys/dev/extres/syscon syscon.c syscon_if.m, sys/dev/fdt simple_mfd.c

Add method for getting of syscon handle from parent device.

If simple multifuction device also provides syscon interface, its
childern should be able to consume it. Due to this:
- declare coresponding method in syscon interface
- implement it in simple multifunction device driver

MFC after:      1 week
DeltaFile
+17-2sys/dev/fdt/simple_mfd.c
+11-0sys/dev/extres/syscon/syscon.c
+9-0sys/dev/extres/syscon/syscon_if.m
+37-23 files

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