[PATCH] aarch64: Update Architecture Extensions documentation

[Nick Clifton]

Hi Andrew,

> Restructure the architecture extensions table, add a new table for architecture
> version dependencies, add missing architecture extensions, and improve some
> extension descriptions.
> 
> Ok for master and 2.42 branch?

Approved for master.  Not approved for the branch.  (This is not really a bug
fix, so it does not meet the criteria for branch commits).

Cheers
   Nick