Add generic 'syscon' driver

Upstream dts for allwinner will require a syscon driver, since the emac node
coming in 4.15 will be using xref to /soc/syscon for configuring the emac
clock. Add a generic syscon driver to attach to /soc/syscon for use by
if_awg, providing basic read/write functionality to consumers.

syscon driver will also be used by arm64 at least for A64+H5 emac/if_awg.

Written by: mmel
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D13295