drm: zte: add tvenc driver support

It adds the TV Encoder driver to support video output in PAL and NTSC
format.  The driver uses syscon/regmap interface to configure register
bit sitting in SYSCTRL module for DAC power control.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>