powerpc: remove genietv board support

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

 - Remove board/genietv/*
 - Remove include/configs/GENIETV.h
 - Clean-up if defined(CONFIG_GENIETV)
 - Move the entry from boards.cfg to doc/README.scrapyard

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