rockchip: video: Add remove() methods

Add remove() methods for EDP and VOP so that U-Boot can shut down the
video on exit. This avoids leaving DMA running while booting Linux which
can cause problems if Linux uses the frame buffer for something else.

It also makes it clear what is needed to shut down video.

While we are here, make rkvop_enable() static.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Squashed in 'rockchip: video: fix taking the VOP device out of standby':
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>