powerpc: remove RPXClassic, RPXlite boards support

Enough time has passed since these boards were moved to Orphan. Remove.

 - Remove board/RPXlite/*
 - Remove board/RPXClassic/*
 - Remove include/configs/RPXlite.h
 - Remove include/configs/RPXClassic.h
 - Clean-up defined(CONFIG_RPXCLASSIC)
 - Move the entry from boards.cfg to doc/README.scrapyard

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