doc: clarify arch support for power platforms

Add specifics as to the level of hardware supported for
power platforms.  Power 8 is the first processor level to
support ppc little endian.  Set the minimum level
for AIX to Power7 for Node version 8 and later.
This will allow the potential to leverage new instructions
and optimizations going forward.  We have spoken to the
AIX team and they agree this makes sense. Earlier processor
levels will continue to be supported for version 4.x and 6.x
on AIX.

PR-URL: https://github.com/nodejs/node/pull/12679


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>