syscon: Introduce kobj and split out fdt bits

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, as is common to the
extres framework.

Reviewed by: strejda
Differential Revision: https://reviews.freebsd.org/D13521