syscon: Introduce kobj and split out fdt bits

[kevans]

Summary

Allow more flexibility by kobj'ifying syscon and splitting out fdt specific bits in preparation of a move to the extres framework. The generic fdt driver has been moved to syscon_generic.c and the fdt requirement has been removed from the syscon interface.

[mmel]

Everything else looks OK for me.

sys/dev/syscon/syscon.c

101

I don't think that we needs/wants default methods for read/write/modify.
Its better to return error, if driver don't implement these.

254

And whats if single device register multiple syscons? I think that we can store ofw node to struct syscon and then use it for search. OF_device_from_xref() is not right approach I think.
see https://github.com/strejda/tegra/blob/master/sys/dev/extres/phy/phy.c#L93
and its usage.

[kevans]

Something like this, perhaps?

• Remove default read/write/modify methods
• Allow creation of syscon node from specific ofw node not necessarily belonging to provider

[kevans]

Re-add context... whoops.

[mmel]

Perfect, thanks.