drm/rockchip: dw-mipi-dsi: add reset control

In order to fully reset the state of the MIPI controller we must assert
this reset.

This is slightly more complicated than it could be in order to maintain
compatibility with device trees that do not specify the reset property.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224125506.21533-24-john@metanate.com