doc: drop PPC BE from supported platforms

We've discussed this with the Power team within IBM and have
agreement that we should drop Power Linux BE for Node version
8 as customers will be using Power Linux LE when using
Node.js going forward.

PR-URL: https://github.com/nodejs/node/pull/12309


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>
Reviewed-By: Jaideep Bajwa <bjaideep@ca.ibm.com>