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/rockchip/Kconfig b/drivers/video/rockchip/Kconfig
index 9267b2878..80e399f 100644
--- a/drivers/video/rockchip/Kconfig
+++ b/drivers/video/rockchip/Kconfig
@@ -35,6 +35,7 @@
 
 config DISPLAY_ROCKCHIP_HDMI
 	bool "HDMI port"
+	select VIDEO_DW_HDMI
 	depends on VIDEO_ROCKCHIP
 	help
 	  This enables High-Definition Multimedia Interface display support.