powerpc: Add some documentation of ISA versions

[Michael Ellerman]

On Mon, 2018-11-05 at 09:01:01 UTC, Michael Ellerman wrote:
> Add some documentation on which CPU versions map to which ISA
> versions. This is all publicly available information, some of it
> already in the kernel source, but it's much nicer to have it all in
> one place.
> 
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/ec2adcd8dd64588cb4bcd1c1e0b863

cheers