powerpc: remove MBX and MBX860T boards support

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

 - Remove board/mbx8xx/*
 - Remove include/configs/{MBX.h,MBX860T.h}
 - Clean-up if defined(CONFIG_MBX)
 - Move the entries from boards.cfg to doc/README.scrapyard

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