rockchip: video: update MAINTAINERS

The video drivers (VOP, HDMI encoder, LVDS encoder, MIPI encoder) for
Rockchip SOCs are self-contained and are mainly impacted by other
changes in the architecture support (e.g. pinctrl, clocking, etc).

Let's add these to the list of files maintained as part of the
Rockchip port.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>