bridge: Elaborate a bit on dumb VGA bridges in Kconfig

It's better if we explain a bit that this pertains to
non-programmable VGA DAC bridges.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302091426.8463-1-linus.walleij@linaro.org