sunxi: change the DE1 video option to CONFIG_VIDEO_SUNXI

The sunxi DE1 video option used to be CONFIG_VIDEO, which has the same
name as the "Enable legacy video support" option in
drivers/video/Kconfig.

Change the option name to CONFIG_VIDEO_SUNXI, which is really used by
Makefile under drivers/video/sunxi/, and defined in sunxi-common.h
when CONFIG_VIDEO is selected before this change. Now CONFIG_VIDEO_SUNXI
selects CONFIG_VIDEO and the usages of CONFIG_VIDEO in sunxi Kconfig and
config headers are all converted to use CONFIG_VIDEO_SUNXI.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>