mpc8xx: remove RRvision board support

This board is still a non-generic board.

Unused code in arch/powerpc/cpu/mpc8xx/video.c should be also deleted
because CONFIG_VIDEO_ENCODER_AD7176, CONFIG_VIDEO_ENCODER_AD7177,
CONFIG_VIDEO_ENCODER_AD7179 are not defined any more.

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