MFC r303087

Add support for the SID (Security ID Module) on Allwinner A10 and A20.
The rootkey is burnt at production and can't be changed, thus is can be used
as a device unique ID or to generate a MAC address (This is was u-boot does).
The rootkey is exposed as a sysctl (dev.aw_sid.<unit>.rootkey).

Reviewed by: jmcneill
Differential Revision: https://reviews.freebsd.org/D6383