drm/hisilicon: Add designware dsi encoder driver

Add DesignWare MIPI DSI Host Controller v1.02 encoder driver
for hi6220 SoC.

v9: Fix module compile error.
v8: None.
v7:
- A few regs define clean up.
v6:
- Change "pclk_dsi" to "pclk".
v5: None.
v4: None.
v3:
- Rename file name to dw_drm_dsi.c
- Make encoder type as DRM_MODE_ENCODER_DSI.
- A few cleanup.
v2:
- Remove abtraction layer.

Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Signed-off-by: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Signed-off-by: Andy Green <andy.green@linaro.org>