powerpc: remove RPXsuper board support

Enough time has passed since this board was moved to Orphan. Remove.

 - Remove board/rpxsuper/*
 - Remove include/configs/RPXsuper.h
 - Move the entry from boards.cfg to doc/README.scrapyard

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>