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/Makefile b/drivers/video/rockchip/Makefile
index c742902..cd54b12 100644
--- a/drivers/video/rockchip/Makefile
+++ b/drivers/video/rockchip/Makefile
@@ -9,6 +9,6 @@
 obj-y += rk_vop.o
 obj-$(CONFIG_DISPLAY_ROCKCHIP_EDP) += rk_edp.o
 obj-$(CONFIG_DISPLAY_ROCKCHIP_LVDS) += rk_lvds.o
-obj-$(CONFIG_DISPLAY_ROCKCHIP_HDMI) += rk_hdmi.o ../dw_hdmi.o
+obj-$(CONFIG_DISPLAY_ROCKCHIP_HDMI) += rk_hdmi.o
 obj-$(CONFIG_DISPLAY_ROCKCHIP_MIPI) += rk_mipi.o
 endif