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