drm/rockchip: dsi: add dual mipi support

Add the Rockchip-sepcific dual-dsi setup and hook it into the VOP as well.
As described in the general dual-dsi devicetree binding, the panel should
define two input ports and point each of them to one of the used dsi-
controllers, as well as declare one of them as clock-master.
This is used to determine the dual-dsi state and get access to both
controller instances.

v6:
  handle master+slave component in dsi-attach
v5:
  use driver-internal mechanism to find dual dsi slave
v4:
  add component directly in probe when adding empty dsi slave controller

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001123845.11818-8-heiko@sntech.de