Sunxi current a20 secondary init

This is a follow-on from the marsboard a20 changes from the previous pull-request.
Note: The diff includes those changes.

This adds:
* Code to power-on and keep the second A20 core in a waiting loop.
* This is important for debugging - JTAG tools require all cores you wish to debug to be powered-on.
* Modified smp-boot API (need to update Linux - not done yet).
  - Check if CPU1 is held in reset, if not, just set the boot_addr and perform a "sev" or a write of (2) to cpucfg->event_in
  - This has been tested with a different (non-Linux) SMP kernel