rockchip: video: introduce VIDEO_DW_HDMI and select for Rockchip HDMI

Instead of having drivers/video/rockchip/Kconfig point outside of its
hierarchy for dw_hdmi.o, we should use a configuration-option to
include the Designware HDMI support.

This change introduces a new config option (not to be selected via
menuconfig, but to be selected from a dependent video driver's
configuration option) that enables dw_hdmi.o and selects it whenever
the HDMI support for Rockchip SoCs is selected.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/video/Kconfig b/drivers/video/Kconfig
index 446cca9..61dfed8 100644
--- a/drivers/video/Kconfig
+++ b/drivers/video/Kconfig
@@ -619,4 +619,13 @@
 	  CONFIG option. See the README for details. Drives which have been
 	  converted to driver model will instead used CONFIG_DM_VIDEO.
 
+config VIDEO_DW_HDMI
+	bool
+	help
+	  Enables the common driver code for the Designware HDMI TX
+	  block found in SoCs from various vendors.
+	  As this does not provide any functionality by itself (but
+	  rather requires a SoC-specific glue driver to call it), it
+	  can not be enabled from the configuration menu.
+
 endmenu