altos/chaoskey: Use SoC device ID as serial number

altos/chaoskey: Use SoC device ID as serial number

To make manufacturing these devices tractable, we don't want to
require a custom firmware load for each device, but we still want a
unique serial number. Fortunately, the SoC provides a 96-bit ID which
we can use.

Signed-off-by: Keith Packard <keithp@keithp.com>