EP88x: remove remainders of dead board

Commit 1b0757ec deleted the EP88x entry from boards.cfg file.
But it missed to remove include/configs/EP88x.h and board/ep88x/.

This commit removes them and adds EP88x to README.scrapyard.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Wolfgang Denk <wd@denx.de>