powerpc: remove ADS860, FADS823, FADS850SAR, FADS860T support

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

 - Remove include/configs/{ADS860.h,FADS823.h,FADS850SAR.h,FADS860T.h}
 - Cleanup defined(CONFIG_ADS), defined(CONFIG_MPC823FADS),
   defined(CONFIG_MPC850SAR), defined(CONFIG_SYS_DAUGHTERBOARD)
 - Remove the entries from boards.cfg

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