aw_sid: Add nvmem interface

Rework aw_sid so it can work with the nvmem interface.
Each SoC expose a set of fuses (for now rootkey/boardid and, if available,
the thermal calibration data). A fuse can be private or public, reading private
fuse needs to be done via some registers instead of reading directly.
Each fuse is exposed as a sysctl.
For now leave the possibility for a driver to read any fuse without using
the nvmem interface as the awg and emac driver use this to generate a mac
address.