drm: zte: add initial vou drm driver

It adds the initial ZTE VOU display controller DRM driver.  There are
still some features to be added, like overlay plane, scaling, and more
output devices support.  But it's already useful with dual CRTCs and
HDMI monitor working.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 483059a..a91f8ce 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -223,6 +223,8 @@
 
 source "drivers/gpu/drm/mediatek/Kconfig"
 
+source "drivers/gpu/drm/zte/Kconfig"
+
 # Keep legacy drivers last
 
 menuconfig DRM_LEGACY